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They wouldn't necessarily be the best species for the size of tank you have but IMO the dwarf cory species (i.e. Corydoras pygmaeus or habrosus) are the most active and interesting to watch. Bandit cories (C. metae) is another one of my favorites but I don't think you could go wrong with any cory species.
I agree, all cories are adorable, but ..... for sheer manic activity it is hard to beat albinos. They will go from being gathered in a nice calm group to zooming all over the tank, just missing (usually) each other, plants, other fish, large solid objects, etc. Seriously just go to a lfs and watch them for a bit. Oh, and for some reason, i have never seen other fish bothered by all the commotion.