I'm fairly broad with natures; I'll take any nature thats pretty good. For example, my swellow is timid which raises his speed but lowers his attack, which is bad in theory. But I keep it because honestly speed is a good stat for swellow, and if it cuts into attack then so be it. Its not like its modest or anything. By that logic, feel free to have hasty, jolly, naive, lonely, naughty, or any of the neutral natures for your crobat. If you want ideal though then it would be jolly