My ever-evolving supplement regimen is largely a factor of my current situation, my particular goals at the moment, and other things individual to me, but maybe some of you will find the logic behind it helpful:
I take 2 standard doses of whey protein 2x/day. This is mainly because I'm currently eating in a calorie deficit and it's impossible (for me, at my age, at my activity level) to get adequate protein from food alone while doing this. Whey as opposed to other sources because it has an excellent amino acid profile. Some others may be better, but because whey protein was until recently considered a waste product of dairy production, it's quite affordable.
To the first protein shake of the day, around lunch time, I add:
5g of creatine monohydrate. I do this partly for its neuroprotective properties, but mainly because I lift and creatine is crucial for supplying energy to muscles. I wouldn't be doing this on a more complete diet, because I eat meat (and other animal products) exclusively, and I get plenty of creatine from my diet if I'm eating ad libitum, but I'm not currently doing that.
20g of collagen peptides. I will probably cut back on this one, if not stop it, when I'm back on a full diet, which should provide adequate amounts of collagen. I take it for joint health. It's also a good source of protein, so I count it toward that macro, but I think its amino acid profile is a bit inferior to that of whey protein.
2g of taurine. Its demonstrated benefits are too many to list here but you can easily google them. Muscle building is not one of them - it's not involved in muscle protein synthesis at all.
4g of leucine. Leucine is an amino acid, uniquely important to muscle protein synthesis because of its role in MTOR activation. I may discontinue when I'm back on a full diet.
With dinner I take:
500mg of a 2:1:1 ratio of branch-chain aminos, which includes more leucine. Branch-chain aminos together act synergistically to promote muscle protein synthesis, and I want as much of that as I can get from my protein-rich but fairly small meals that I currently eat once a day.
5,000 IUs of vitamin D3 because it's winter and I'm not getting a lot of sun. I take it with dinner because it's a fat-soluble vitamin. I will stop doing this next May at the latest, until next winter.
Before bed, I take:
SlowMag, a highly bioavailable magnesium chloride supplement formulated to be absorbed slowly to minimize digestive side-effects, as my main non-dietary means of replacing used/lost magnesium. Magnesium is a massively important mineral that I'm probably not getting enough of especially on my limited diet, and you might consider googling the long list of important functions it serves in the body.
Magnesium L-threonate, formulated specifically to penetrate the blood-brain barrier. It's been shown to improve memory and cognitive function specifically in men over 40, which includes me, and I've been having some memory issues (cognition is still fine).
A fish oil supplement to improve my omega-3 to omega-6 ratio. I may discontinue this one when I'm on a full diet, if I manage to incorporate enough seafood into it. Google up this ratio and its significance if you're curious and not familiar.