Zombies: Dawn of the Dead - the original 1978 UK theatrical release imo was the fantastic, which, IIRC, had some of the less believable zombie/biker scenes cut, especially the gut eating scene, which ever-so-slightly spoils an otherwise perfect movie.
The remake was OK, but not a patch on the original. Shaun of the Dead on the other hand was excellent!
Day of the Dead (1985) is also a great film until the silly & unbelievable death scene of Captain Rhodes at the end.
But the best is the original Night of the Living Dead.
I haven't seen Diary of the Dead yet.
I love I Walked With a Zombie (the 1943 version), but that film belongs in the earlier voodoo curse interpretation of the zombie genre.
BTW, 28 Days Later is not a zombie film. If we're going to include that then I would nominate Romero's The Crazies as being much better.