If you are big on top, you may need some extra support, underwear tops can work well and look great. Halter-top bikinis can be a good choice by offering support, while providing some sexy cleavage too. Look for styles that are banded around the midriff and can be tied in back and around the neck. This style will give you some lift and will allow you to make adjustments for a personal fit.
Look for one-piece swimsuits with some structure in the bust, such as soft foam cups or at least, a shelf bra. Straight cut bust styles such as a classic tank can look clean and sporty. Wide shoulder straps will also be much more comfortable than thin, spaghetti straps.
You should avoid strapless bandeau tops and those tiny, tri-tops that offer minimal coverage. Make sure to get a suit where you can order the top and bottom sizes separately.