I can 100% vouch for CheapAir, have been using them almost exclusively for booking flights all over the place for the past while. Their UX is a lot better than some other travel websites, and customer support is pretty on point.

I'll have to give the other ones a shot, see if they work out - the only downside to CheapAir is they don't support some carriers (RyanAir isn't available through them last I checked) which can make some routes in Europe a bit more expensive.