The common ones found at most aquariums are :
M. Boesmani
M. Lacustris (Turqouise rainbow)
M. Herbertaxelrodi
M. Praecox (stay small and most of what you see in stores is bad examples - washed out/overbred)
M. Splendida Inornata (I had a pair and they were AWESOME! Really colored up when displaying to each other).
G. Incisus
G.
Frankly, your best bet would be to call stores local to you to see what they carry and then do your research on the website since that will tell you how big they get. Most will get to 4" or more. If the store says they have the generic 'Australian Rainbowfish' stay away. These are normally mass raised fish in Florida ponds that are a mish-mash of different strains and aren't that good looking.
If you want to geek out some more, a good resource for US based rainbow fish keepers is
www.rainbow-fish.org
I kept rainbows for a couple of years and then decided I wanted to keep tetras. I have had tetras for about 1 year and while I like schooling fish they aren't nearly as interesting as rainbows. When rainbows start displaying to each other they take on some beautiful colors and are a sight to see. I am definitely going back at some point.
Which, of that huge list, would you recommend I consider?
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