We've had 'em all. My personal fave was the guinea pig, like the gang sez, friendly, big enough to hold, they have enough personality to be interesting but not so much that they are a pain. Pretty easy animals to own/care for.
Hamster/gerbil, they say if you want one get a hamster, they like to live alone, more than one get gerbils, they like group life. A couple of gerbils is one of the easiest pets you can own, not too demanding of the owner's time, kinda fun to watch. Well, it's company anyway. They do have different personalities/temperments, from friendly to nasty, and remember, they are nocturnal. If you stay up all night computing like I do, it's nice to hear their little wheel & chirping, but they'll wake the kids in the kids bedroom.
Some people really like mice. I didn't but then when we got one, it was really sweet. More personality than the gersters, actually. But you will get a lot of stupid stories when you tell people what your pet is.
All are better company than fish - I like fish, but they're not much in the companion line. As for expense, the animal isn't much, the accessories can add up. The rage nowadays is for the fancy tube "habitat" setups, which can run you a couple hundred dollars if you go whole hog. You can get perfectly good cages for a lot less. And check the want ads/garage sales in the fancy suburbs when junior's fancy hamster dies, they want to unload the cool habitats for nothing. We got ours out of a neighbor's garbage.
Also, when your gerbil gets sick you don't have to spend hundreds taking it to the vet. You just get another one. People who would be shocked to see a dog or cat suffer are perfectly fine with gerbil-replacement. Although I have taken them to the vet, which is why I know how much it costs to set a gerbil broken leg. But if it were just me, I would have let it go to its little gerbil-valhalla and swung by the pet store.
I don't like rabbits.