I'm with Phasma in that I hardly ever swim, ever. Some very unpleasant swim lessons back in middle school taught me to swim but also taught me to dislike swimming.
I guess if I had to, it would be swim trunks, or more realistically, shorts appropriated to be swimwear, since I never actually make plans to go swimming.
As for the women, however, definitely the two-piece. If we're all agreeing to unearth our pervy sides here, then for me, I like some tease to my fanservice. Something as skimpy as a V suit leaves too much exposed and not enough hidden for the imagination to wonder about.