What I regret are all the bikes I've sold and should have kept.... Honda Cub, TW200, the list goes on.
My least favorite bikes are the ones without motors...
...I'm even hesitating on the 450 part!!! 
Chineese ones. I have a CRF150F in addition to my 250 and I really like it as a play bike. Easy to work on and reliable (for the most part). I don't have any bikes that I hate really, that are in the fantastic 4 & KTM. China bikes are POSes though. Really.
