This is tough to choose. My favorite would probably be my female veil marble angelfish. She was part of a pair and spawned many times until the male became aggressive and I moved him to a different tank. She's in my bedroom in a tank that's in front of a window and she likes watching me whether I'm inside or out. She has quite a bit of personality and is mellow for an angel.
I also find myself smiling whenever I watch my yoyo loaches since they are such active, acrobatic, inquisitive guys and seem so joyful darting around the tank, cruising the currents and exploring everything and everyone. They come up to the surface when I feed them and if I hold the food they'll eat it from my fingers. It's so funny seeing 5-7 of them perfectly vertical doing their darnedest to eat what I'm holding before anyone else can get it.
My 5 congo tetras. Their colors are beautiful. They're fun to watch as they chase one another. When I'm working in the tank and happen to have one hand just above the surface they will jump up and "kiss" my fingers, anticipating food, since I sometimes hand-feed them. They don't bother the Hengel's rasboras.
Kuhli loaches do the craziest things and probably are the fish I look the most forward to seeing in my tank. I don't always get to see them because they like to hide, so it makes seeing them out and about that much better.
Love my clown loaches. Has anyone else ever herd the clicking noise they make when that are happy. Its crazy I never thought that fish even made noises. They are amusing, they fight with my pleco to get some cucumber. Thats fun to watch, niether back down.
Oh yeah, if we go to the salt side of things, my Scooter Blenny is one of my all time favorites. He's like a little puppy dog in the tank the way he follows you around and stands up to try to get a better view of what you're doing.
On the fresh side, there are so many that I really like, but I guess I'd have to say my snowball plec is way up there.