OP should've made this a poll post so we could actually see what suit has the most preference xD
Well, regarless of that I'll state my favorites and why I use them.
1) Dark Samus alt. This one if my absolute favorite because in Brawl my favorite alt. was her Dark Suit one. It just looked so cool to me because of how Samus looked in a suit I've never seen before (I was a scrub back then with no Wii/GC). Plus Dark Samus was a character that I was REALLY hoping for in this game. So, this is the next best thing. Plus the sleek black design is just so awesome and edgy!
2) ALL THE OTHERS!!! LOL
I really have no other preference besides the Dark Samus one. Its the only one I'll use in tourney actually. I play most comfortably with it (could be a psychological thing; Game Theory anyone?). The other suits I only really use at random when playing friendlies on/offline; or in training. I do really like the Gravity Suit and Dark suit a lot though. In no particular order. They're just my favorites because purple is my favorite color, and the Dark Suit still looks pretty sick compared to Brawl. Oh but how I wish Samus could have all her actual suits kinda like in PM. That would be amazing. Oh well. I'm still grateful with these sick alts nonetheless.