Rosy barbs. They are always moving and have loads of character. They always come up and inspect everything. They eat out of my hands and they dart everywhere. Added to that, the male ones are the most beautiful reddish/purple colour. I would say hands down these are my favourite fish.
I have to go with rosy barbs as well. When I had 4 adults they would stick together and dart all over the tank. When I did a water change or do anything that inloved me sticking my arm in the tank they would try and eat the freckles off my arm, thinking it was food. 3 of them died (), but had just left behind a mess of babies! So now I have 6 babes and 1 adult. 2 of the babies follow the adult around like a lost puppy, 2 baby males are constantly fighting, and the other 2 think they are platies and spend all day with my baby platies. Silly barbs, trix are for kids! I mean... yeah.
I agree with Wilder about the shoaling thing. My neons only shoal after I've just done a water change or if I've accidently sucked one up the siphon (it happens ok - they swim up to investigate the tube, stupid fish). After they've recovered they split up again.
I have a school of 7 neons in my 30 gal and they look really pretty. I didn't notice how bright and colorful they were until I recently just added some bright live plants and a black background. I could stare at them all day! =)
I once saw a huge school of silver dollars in a large tank in a public aquarium. Just beautiful! I think a large school of congo tetras would be pretty impressive. I The nicest school I've had personally is my current one, 16 neons. When they do school up, they look great. Mostly they wander in smaller groups of 4-6.