Fish are terrible pets because the few joys of owning a fish can be replicated without any of the negatives by having a fish tank screen saver for your computer instead.
I had a guinea pig for awhile as a kid, and he was the worst. He would occasionally run in circles, blowing bedding (possibly piss covered) out of the sides of his cage and making weird noises. When I took him out to play with him, he would just sit there like a dumbass and maybe shit on the carpet. He was basically like a rabbit, only not as cute.
I had a hamster that was a little more entertaining, but I actually think the hamster's accessories were more exciting than the hamster itself. The cage has all those cool tubes (which are all equally great vessels for hamster piss) and the ball you can put them in is sort of hilarious.
Now that I'm older, I've kind of decided that all rodents are simply to stupid to be good pets. Their brain basically governs eating, fucking and shitting and attempts to get them to do things outside of that are mostly fruitless. In retrospect, I shouldn't have expected that much from animals whose species survival strategy consists of reproducing so fast that predators are to full to eat them all.
You can't really pet reptiles so those kind of stink.