BEST: Blade Runner. This was one of the first times I had seen product placement in a sci-fi film, and I thought it worked perfectly. The fact the the city seemed so dark and decaying, contrasted with the bright sunny Coke ads etc. seemed to be more of a statement than actual placement. It doesn't distract me at all...only heightens the realism for me.
WORST: Spider-Man, for the Dr. Pepper plug (does anyone actually drink that stuff?) when Peter is honing his webshooting skills in his room. And while not stictly "product placement", who can forget the unnecessary appearance of Macy Gray singing her song?
Lastly, I'll need confirmation on this one since I didn't actually see the film, but I understand the "the Cat in the Hat" came to a screeching halt to promote a amusement park vacation or something. Anyone?