Depends on the time of day or what i have done to the tanks.
My Kuhli loaches have some pretty fun antics. If i turn off the filter for an hour they love to swim up the side of the tank. I have some slow motion action shots of them bending into question marks and springing into the air. They make some intricate displays when three or four are all doing this together.
My Harlequin Rasboras can be enticed into breeding displays if i drop the water temperature a little with WC
. I've never successfully bred them though.
My latest favorite to watch is a Betta whose tank i set up in my room. He goes about his business while i am sitting still in my room but the second he sees me stand up he is like a dog running laps along a fence. He gets very excited when i approach. When he is bored he often decides to headbutt his Ramshorn snail tankmates. His other trick is to knock the thermometer into the water.
Goldfish are also extremely social fish if you have room for them. I love handfeeding mine portions of fruit.