I really like Gnaw and Coco, those are my favorite I think. Though I've found with Coco I either do really well, or am completely useless in a match, heh.
Voltar, Clunk, and Lonestar I like as well. Voltar you can do kind of cheesy rushes against the bots which they can't respond fast enough to do anything about it - though that is less effective against players.
Yuri I want to like, but he's just so hard to play well. I just die a lot with him, heh.
As for bots gaining money, they probably do gain it through other means, but I don't think they go out of their way to get solar, they make quite a bit from their passive gains.
Edit: A leveled up Leon is painful to face. I don't really like playing as him, but a good Leon and a good Froggy G have been some of the toughest I've gone against. They can really bring your health down fast, and both have good escape abilities, and good ways to gank you as you try to run.