I do hate to call the 28 Days/Weeks movies "Zombie movies", but I suppose they're close enough to pose a recommendation.
Obviously Romero's Night, Dawn, Day, Land and Diary. Each have their merits, with Land and Diary being the weakest.
Lucio Fulci's "Zombie" is my favourite zombie flick, bar none, and pretty much all because of a couple of shots during.
The "Dawn of the Dead" remake was a very nice surprise. I still think it's a great zombie movie.
Again, not exactly a Zombie movie per se, but "[REC]" fits the bill and is extremely recommended.
"Zombie Strippers" is actually very good, with a lot of fun humour and low-budget CGI gore pulled off very well. The physical effects are top notch for such a low-budget movie. It's from the guy who directed "The Slaughter" - a low-budget Evil Dead rip off that managed to rise above other direct-to-DVD dreck despite a lot of extremely ropey CGI gore.
Shaun of the Dead is top class. You really do need to see it, but I'm sure you already have.
Fulci's "The Beyond" and "City of the Dead" are Italian classics. Very good and atmospheric, but as far as actual zombie mayhem goes they're nothing on "Zombie". While not strictly a Zombie flick, "The House by the Cemetary" is also amazing. The final scenes make it worth it.
Oh, and Peter Jackson's "Brain Dead". If you haven't seen that film, you haven't lived.