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...
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.