I've had a blast at it in the past a few nice bikes out there I had a Kawasaki hated it rusted and the gearbox was a nightmare , 2 stroke fantastic but noisey and can bitch to maintain especially 250s 125s can be fantastic fun nice and light , depends on your size and build, but Honda as have been mentioned the 4 stroke seem robust and bomb proof , I love a good Husqvarna to be fair though. Lots to look at but depends what your doing with it , I've a full license so engine size is irrelevant, but I've been thinking of getting a 125 to get out on suprisingly fun and cheap .
Hope that helps at