One-piece. I feel a little too self-conscious about wearing two-piece swimwear although I like the look of bikinis. That's mostly cause while I do like the thick amounts of dark hair on my stomach, I don't like getting attention for showing it off, but also don't wanna shave it just for that. I might feel brave enough for a bikini one day. But thought I'd practice my confidence with crop tops and tied blouses during summer first. It's scary to be gnc!
Currently I have a black one-piece in halterneck style from Ellos (Swedish webshop, similar stuff as H&M) that I wear as it is without breast forms or any padding in the chest area. I tried to find a decent swimsuit for breast forms but to no avail. Everything either too ugly (I still have the horrid mental image of a mint green thing with white polka dots, that I saw at an online shop... not my style!) or with straps that would easily fall off my shoulders (I don't need to additionally worry about accidental flashing). So I gave up on that after much frustration.
The swimsuit I wear has no cups, just stretchy fabric all over, so it looks neat over my flat chest. Going flat with the swimwear does make me dysphoric, but it's not so bad that I can't disassociate from it while swimming (and mostly hiding under water anyway), which I do about once a week at a local pool, for exercise.
When I have breasts again, I'll probably be wearing the same kinds of swimwear in general, but just feel a million times better in them.