I have a strong preference for tankinis. It's ironic, considering my midsection is my biggest trouble zone and needs to be covered! But, since I'm so short, a tankini will cover it all up and show no skin, and something about having the line there across my tummy confuses the eye so you don't notice it.
I like to go with a patterned top, and plain black bottoms. That's because I find that the bottoms tend to wear out fastest - from sand or pool cement rubbing when you sit. If you go with a two-piece, you can then replace the bottoms if they get worn out before the top.
My favorite shape for the tankini top is halter style. It minimized my broad shoulders, and you can adjust the ties to keep things from sagging. Underwire is good too. I don't care so much for anything with pads (I don't need the bulk, but also, even small ones tend to deteriorate over time, move around, etc), or for shelf bras, because they give you the uniboob look. But I do have one shelf bra tankini from VS that I wear if we're doing active things - it squashes things down like a sports bra.
Colors: black is always safe, but color is more flattering. I like blues and turquoise - anything that complements my hazel eyes. Bear in mind that if you pop a sundress over it, it's nice if the suit complements the dress. A fun color can be nice for this.
Lands' End has a lot of nice tankinis to choose from. Victoria's Secret does too, so I recommend checking that site as well. Otherwise, best strategy is to hit up a department store and try on EVERYTHING. It's very hard to know which suit is going to look best, until you try it on.
I think Angie did a post on cover-ups before?
