The thing that it completely depends on is the lighting you have in your tank, if you have a reef supporting SPS corals then many clams are available.
Maximas are the MOST sensative and fragile, but have intense colouring.
Croceas- Grow to about six inches, but always look best from the top, and are the most light demanding.
I would say are the next most sensative but by no means as bad as the Maximas. They are also very slow growing.
Derasas require less light but not the least, they can tolerate lower wattage halides and T5s. I have seen them at around 14inches in the home aquarium and would look great in a 125. They grow faster than croceas. Derasas are very nice pattern wise but generally are not the most colorful.
Squamosas require less light as well and grow upwards of 20 inches at times but are also slow growing because they devote much of their growth to scutes on the sides of their shell. MY new squamie is a nice specimen with lots of color, and they look equally awesome from the top or the sides.
Gigas can handle the least light and I think are AWESOME, they also grow about 8 times faster than any other species. They are true giant clams and can grow upwards of 3 feet. The biggest I have seen in a home aquarium is around 27 inches, in the wild a big 3.5 footer. They are hardy and I think have very nice patterns and blue spots from the side.