Best:
Night of the Living Dead
Dawn of the Dead (original, not remake)
Day of the Dead
Evil Dead (original)
Shaun of the Dead
Good/Fun:
Land of the Dead
28 Days Later
Army of Darkness (especially fun is to watch the commentary by Bruce Campbell and Sam Raimi)
On a side note, I went to see Romero's Diary of the Dead two weeks ago- and it's not very good at all. Much as I love the man's movies, I can not recommend that one in the least.
[EDIT- Can't believe I forgot to include the Resident Evil movies. I'd put the first Resident Evil in the first category, or near enough- it was pretty good. The second I'd place in the second category, and the third I didn't care too much for at all.]