I liked Jackie Brown too. Not sure which to pick though, that or Pulp Fiction, as they're both great in their own way.
I liked Reservoir Dogs. I think Pulp has the edge for me though.
I've yet to see Grindhouse.
I enjoyed Kill Bill too. I think I'd rate it bellow the others simply because the storyline was extremely basic (although I accept it's not about that.)
Overall I find choosing favorite films difficult, as I see what each is trying to do, and enjoy it in different ways. (Same with the Evil Dead films by the way.)
Oh another great film written and starring Tarantino - From Dusk till Dawn. He didn't direct it though so I'm not sure it should be included.
Memorable scene. Sigh!