This is always an interesting question isn't it?
Dirtbikes wise, of them all that I've owned or ridden, the '05 YZ125 was the best. Light, fast enough for me, handled great, and let me ride without worrying about it getting away from me.
Vintage wise, my 1980 IT175G always left a special feeling when I rode it and now I own one again I still love it. Not the best bike around but just a fun little beastie.
And just for something out of left field, the 1984 CR500 Husky was a ball. Never really got on with it, but it was grunty, fast, plush and had great suspension for an old twin shocker. I even scored a couple of trophies with that which is a very unusual thing for me!!!
Oh and a special mention for the TT500C. Best looking dirtbike ever in my opinion, and when ridden how it's meant to be (lazy afternoons doing mono and just sitting looking at it), it's just brilliant. But never never ride one in a tight hilly enduro...