I have the Underworks Tri-Top Binder (style 983), small. I'm probably a 34 B bra if that helps.
Gets me very flat, although at the bottom of my breasts there's a bump where they end that can be felt, though not seen except through really tight clothing. Comfortable, I've spent up to 12 hours in it with minimum aching or breathing difficulty. I don't even notice it most of the time. So overall, I'm really happy with it. On the downside, I cannot put it on over my head without it nearly strangling me, the fabric's too stiff. I have to step into it, and since it has to go over wide hips daily it's stretching out a bit. Also, it's a little longer than it needs to be and rolls up over my belly, causing stiff folds that can show through thin shirts. Same problem around the neckline, if the shoulder straps aren't far enough apart in bunches in the middle and that fold is evident through a t-shirt.