I worked at a pet store for nine years. Only going to make a short post since I'm at work.
I've had many rodent pets over the years. I will say it depends on what you want as a pet for suggestions.
Rats are very intelligent (about the same as your average dog). You can train them. But just like dogs they need lots of mental and physical stimulation. All rodents need both but I'd say rats need a ton.
Mice are a bit dumber than rats. But they tend to have more similar personalities across the board rather than with rats are a bit more individuals. That's not saying mice don't have personalities.
Hamsters, it can depend a ton on the breed and their personalities. Robos tend to be the 'nicest' in that they don't bite (at least, I was never bit and one of my main jobs was cleaning the cages a lot haha). Syrians tend to be friendly. I never had much luck with the dwarfs lol. They were almost always neurotic. I did enjoy the squeaking of the djungarians

I used to babysit some chinchillas. So soft but need to be kept cold!
Cannibalism is usually the animals aren't getting enough stimulation, nor enough room, nor food/water.
Most of these I've had as pets. The rats were fun. Tumors aren't though. I'd say, I'd probably get mice again if I got rodents or the like. Come in lots of colors and styles and are generally friendly. Also easy to sex at a young age (only female mice have nipples).