Like JD said -- find a local greenhouse, or a garden center, and talk to them and see what looks good. I'd rather have a short, stocky plant than a big tall one. I always make sure they look healthy too, unless I'm desperate. I start from seed indoors each year, but it seems like I always end up buying a few for one reason or another.
I usually grow several heirlooms and a couple hybrids tomatoes. Celebrity hybrid is a medium-sized reliable red mater that's determinate, disease resistant, and it's widely available. Sun Gold hybrid is an outstanding yellow/orange cherry. I'm not a big cherry eater, but these have a great sweet/tart flavor almost like citrus. Heirloom favorites that are pretty available are Brandywine and Cherokee Purple. Really great flavor on those two.