@onceuponatime has put a bounty of 25 SBD for the first person to compile the list of all participants in his Behind the Mystery Door Contest. Plus there is also 25 SBD prize for a person giving viable suggestion regarding the selection of winners of the contest.

So here are my suggestions.

Below I have compiled a list of all participants along with their reputation and entry post payout. The purpose behind putting reputation and post payout along with contestants is allow @onceuponatime easily analyze data and see if there is some undervalued post that he have missed (missed his hefty upvote). Plus if I were in place of @onceuponatime, I would have looked carefully at the participants with reputation score between 45 and 55.

Why reputation score between 45 and 55

In my experience the steemians with reputation score between 45 and 55 are the struggling ones. They are usually competent and genuine enough to achieve that reputation score, but are not famous like 60+ reputation steemians to earn as much as their content deserve . So I feel when choosing a winner for his Behind the Mystery Door Contest, @onceuponatime should must give priority to these steemians. It will be a huge encouragement boast. Besides what's the point of having a winner from steemians who already have 1000+ steem power under their belts and are comfortably making respectable money as their post payouts.
I am in no way trying to undermine 60+ reputation steemians but I feel the steemians between 45-55 reputation score are the most deserving ones. So at least one winner among three, should must fall in this class. That is why I have also put reputation score in table below.
These are only my suggestions and I leave the rest at your better judgment @onceuponatime.

Sr #	User Name	Reputation	Entry Post	Post Payout
1	@lordneroo	59	https://steemit.com/writing/@lordneroo/no-roads-left-behind-the-mystery-door	$24.64
2	@vanessahampton	54	https://steemit.com/behindmysterydoor/@vanessahampton/unclad-behind-the-mystery-door	$20.59
3	@figuringoutsrn	46	https://steemit.com/funny/@figuringoutsrn/another-man-saving-the-world	$19.50
4	@skiponline	44	https://steemit.com/beyondmysterydoor/@skiponline/beyondmysterydoorajourneyofthemind-v3g7hgedrt	$14.72
5	@alexandraioana26	57	https://steemit.com/behindmysterydoor/@alexandraioana26/behind-the-mystery-door-contest-doorception	$27.12
6	@tkappa	61	https://steemit.com/behindmysterydoor/@tkappa/behind-the-mystery-door-there-s-jabaja	$23.24
7	@allesia	59	https://steemit.com/behindmysterydoor/@allesia/love-story-at-the-castle-behind-the-mystery-door-contest	$16.74
8	@diebitch	58	https://steemit.com/behindmysterydoor/@diebitch/portal	$21.26
9	@sakura1012	53	https://steemit.com/behindmysterydoor/@sakura1012/behind-the-mystery-door-or-a-jouney-to-the-unknown	$24.53
10	@chinyerevivian	45	https://steemit.com/behindmysterydoor/@chinyerevivian/behind-the-mystery-door-the-jewel	$1.27
11	@kindley	47	https://steemit.com/behindmysterydoor/@kindley/behind-mystery-door-endless-choices	$17.64
12	@hrissm	51	https://steemit.com/writing/@hrissm/writing-nowhere-leads-to-somewhere	$20.32
13	@sasho	59	https://steemit.com/sacrifice/@sasho/they-unraveled-the-mystery-of-the-ancient-door-to-hell	$11.39
15	@xabi	53	https://steemit.com/behindmysterydoor/@xabi/my-own-graveyard-behind-mystery-door	$20.43
16	@intelliguy	67	https://steemit.com/behindmysterydoor/@intelliguy/behindmysterydoor-contest-entry-for-oneuponatime-the-end-of-the-road	$27.62
17	@izzynaveda	44	https://steemit.com/behindmysterydoor/@izzynaveda/behind-mystery-door-whole-new-world	$17.58
18	@princefizzy	46	https://steemit.com/behindmysterydoor/@princefizzy/behindmysterydoor-contest-entry-for-onceuponatime-the-end-of-the-road-dd13b27113a73	$19.89
19	@djoi	47	https://steemit.com/behindmysterydoor/@djoi/behind-the-mystery-door	$19.79
20	@zizymena	52	https://steemit.com/behindmysterydoor/@zizymena/the-end-of-the-road-behind-mystery-door-3371cccc1c82d	$17.58
21	@diamondrich	51	https://steemit.com/behindmysterydoor/@diamondrich/behindmysterydoor-contest-entry-for-onceuponatime-the-end-of-the-road-a8163f1901d9d	$7.37
22	@rjunaid12	53	https://steemit.com/behindmysterydoor/@rjunaid12/behind-the-mystery-door-living-the-dream-1e58f94c5386b	$16.99
23	@johnartist	45	https://steemit.com/behindmysterydoor/@johnartist/mystery-behind-the-door-entry-for-the-contest-of-onceuponatime	$18.52
24	@jemmanuel	53	https://steemit.com/behindmysterydoor/@jemmanuel/what-lies-behind-this-door-or-my-entry-to-onceupon-a-time-contest	$18.93
25	@morbyjohn	53	https://steemit.com/behindmysterydoor/@morbyjohn/seeking-the-mystery-of-the-door-entry-for-the-contest-of-onceuponatime	$16.92
26	@roselyn028	50	https://steemit.com/behindmysterydoor/@roselyn028/behind-the-mystery-doors-lies-a-surprise-someone-who-truly-deserve	$19.19
27	@olivia08	50	https://steemit.com/behindmysterydoor/@olivia08/what-lies-behind-this-my-entry-to-onceuponatime	$15.57
28	@old-guy-photos	65	https://steemit.com/behindmysterydoor/@old-guy-photos/behind-the-mystery-door	$24.59
29	@nikosnitza	55	https://steemit.com/behindmysterydoor/@nikosnitza/behind-the-mystery-door	$4.66
30	@aafra	45	https://steemit.com/behindmysterydoor/@aafra/behind-the-mystery-door	$0.00
31	@elisea	43	https://steemit.com/behindmysterydoor/@elisea/the-door	$17.71
32	@madushanka	58	https://steemit.com/behindmysterydoor/@madushanka/the-end-of-the-road-behind-the-mystery-door	$2.81
33	@jet-line	34	https://steemit.com/behindmysterydoor/@jet-line/behind-the-mystery-door-try-luck	$0.00
34	@alinak15	59	https://steemit.com/behindmysterydoor/@alinak15/put-your-face-on-my-window	$20.00
35	@rlt47	53	https://steemit.com/behindmysterydoor/@rlt47/7s3poe-my-entry-behind-the-mystery-door-contest	$17.01
36	@ryunamist	52	https://steemit.com/behindmysterydoor/@ryunamist/contest-entry-behind-the-mystery-door-afterlife	$17.81
37	@salmanbukhari54	58	https://steemit.com/behindmysterydoor/@salmanbukhari54/i-saw-you-there-behind-the-mystery-door-contest	$32.06
38	@sili	49	https://steemit.com/behindmysterydoor/@sili/behind-the-mystery-door	$17.72
39	@josediccus	60	https://steemit.com/behindmysterydoor/@josediccus/the-end-of-the-road-and-behind-the-mystery-door-contest-sometimes-the-mysteriously-behind-the-door-is-filled-with-futher	$18.65
40	@komal	58	https://steemit.com/behindmysterydoor/@komal/behind-the-mystery-door	$21.18
41	@bluemoon	55	https://steemit.com/behindmysterydoor/@bluemoon/the-end-of-the-roadan-idea-from-a-contest	$24.73
42	@nsbachurin	50	https://steemit.com/behindmysterydoor/@nsbachurin/demon-s-room	$17.83
43	@ruth-girl	65	https://steemit.com/behindmysterydoor/@ruth-girl/the-key-to-the-mystery-door-dark-fiction	$30.60
44	@geke	65	https://steemit.com/poetry/@geke/behind-the-door-original-poetry-contest-entry	$3.58
45	@unshakeable	49	https://steemit.com/behindmysterydoor/@unshakeable/behind-the-mystery-door	$19.24
46	@anna-mi	56	https://steemit.com/behindmysterydoor/@anna-mi/behind-a-mysterious-door-competition-from-onceuponatime	$17.89
47	@obaidb2	53	https://steemit.com/behindmysterydoor/@obaidb2/behind-mystery-door-65d9a9beb51ac	$18.12
48	@skapaneas	69	https://steemit.com/art/@skapaneas/behind-the-mystery-door	$34.85
49	@kamchore	51	https://steemit.com/behindmysterydoor/@kamchore/behind-the-mystery-door-solved	$19.04
50	@evimeria	59	https://steemit.com/behindmymysterydoor/@evimeria/behind-the-mystery-door-my-entry-tie-simplicity-behind-the-mystery-road	$18.38
51	@seesladen	49	https://steemit.com/behindmysterydoor/@seesladen/behind-the-mystery-door-a-trip-to-the-endless-beginning	$17.75
53	@aumsong	56	https://steemit.com/behindthemysterydoor/@aumsong/behind-the-mystery-door	$18.47
54	@nikosnitza	55	https://steemit.com/behindmysterydoor/@nikosnitza/behind-the-mystery-door	$4.66
55	@jelumanao	43	https://steemit.com/behindmysterydoor/@jelumanao/what-lies-behind-this-mystery-door-or-onceuponatime-5f85430c2c2c	$16.11
56	@awakeningartist	64	https://steemit.com/story/@awakeningartist/behind-the-mystery-door-a-contest-by-onceuponatime	$19.72
57	@mumin007	51	https://steemit.com/behindmysterydoor/@mumin007/what-is-behind-the-mystery-door	$0.44
58	@wiralhokseumawe	51	https://steemit.com/behindmsterydoor/@wiralhokseumawe/behind-the-mistery-door-gua-door-7-laweung-06c2547a6c91f	$1.61
59	@hazelalex	48	https://steemit.com/behindmysterydoor/@hazelalex/behind-the-mistery-of-door-a-scary-story	$1.37
60	@olawalium	56	https://steemit.com/contest/@onceuponatime/the-end-of-the-road-and-behind-the-mystery-door-contest#@olawalium/re-onceuponatime-the-end-of-the-road-and-behind-the-mystery-door-contest-20180402t215507062z	$0.42
61	@tulhachouhdry	46	https://steemit.com/contest/@tulhachouhdry/re-onceuponatime-the-end-of-the-road-and-behind-the-mystery-door-contest-20180331t102526684z	$0.20
62	@sarmad117	45	https://steemit.com/contest/@sarmad117/re-onceuponatime-the-end-of-the-road-and-behind-the-mystery-door-contest-20180331t094958968z	$0.40
63	@flashfiction	57	https://steemit.com/contest/@flashfiction/re-onceuponatime-the-end-of-the-road-and-behind-the-mystery-door-contest-20180402t213736686z	$0.42
64	@karenmckersie	70	https://steemit.com/contest/@karenmckersie/re-onceuponatime-the-end-of-the-road-and-behind-the-mystery-door-contest-20180402t103928216z	$0.52
65	@alexaventuria	51	https://steemit.com/contest/@alexaventuria/re-onceuponatime-the-end-of-the-road-and-behind-the-mystery-door-contest-20180331t100850533z	$0.20
66	@lifenbeauty	58	https://steemit.com/contest/@lifenbeauty/re-onceuponatime-the-end-of-the-road-and-behind-the-mystery-door-contest-20180331t115056896z#@onceuponatime/re-lifenbeauty-re-onceuponatime-the-end-of-the-road-and-behind-the-mystery-door-contest-20180331t123351799z	$1.01
67	@yahialababidi	59	https://steemit.com/contest/@yahialababidi/re-onceuponatime-the-end-of-the-road-and-behind-the-mystery-door-contest-20180406t225515688z#@onceuponatime/re-yahialababidi-re-onceuponatime-the-end-of-the-road-and-behind-the-mystery-door-contest-20180407t161408887z	$1.00
68	@rosatravels	66	https://steemit.com/contest/@rosatravels/re-onceuponatime-the-end-of-the-road-and-behind-the-mystery-door-contest-20180404t140407583z#@onceuponatime/re-rosatravels-re-onceuponatime-the-end-of-the-road-and-behind-the-mystery-door-contest-20180406t173039213z	$0.76
69	@lavanyalakshman	50	https://steemit.com/contest/@lavanyalakshman/re-onceuponatime-the-end-of-the-road-and-behind-the-mystery-door-contest-20180331t170249521z#@onceuponatime/re-lavanyalakshman-re-onceuponatime-the-end-of-the-road-and-behind-the-mystery-door-contest-20180331t172846359z	$0.39
70	@jasonabejero	47	https://steemit.com/contest/@jasonabejero/re-onceuponatime-the-end-of-the-road-and-behind-the-mystery-door-contest-20180406t033013078z#@onceuponatime/re-jasonabejero-re-onceuponatime-the-end-of-the-road-and-behind-the-mystery-door-contest-20180406t191515458z	$0.31
71	@faizan07	48	https://steemit.com/contest/@faizan07/re-onceuponatime-the-end-of-the-road-and-behind-the-mystery-door-contest-20180331t100802210z#@onceuponatime/re-faizan07-re-onceuponatime-the-end-of-the-road-and-behind-the-mystery-door-contest-20180331t101419355z	$0.20
72	@hanief	47	https://steemit.com/contest/@hanief/re-onceuponatime-the-end-of-the-road-and-behind-the-mystery-door-contest-20180331t095936521z#@onceuponatime/re-hanief-re-onceuponatime-the-end-of-the-road-and-behind-the-mystery-door-contest-20180331t100641498z	$0.20

Hope @twiceuponatime finds my suggestions practical and I earn my bounty:)

Over and out!!!