First and foremost, everything in the story was done on a realistic manner. As in "Oh no what do we do, getting chased by zombies! Hey! Lets hold up in a MALL! With only a few weapons!". In I Am Legend, his 'fortress' was excellent and well protected, plenty of weapons and ONTOP of that the explosive devices and lights outside. Nothing bugs me more in zombie flick than stupid unbelievable actions or just people acting dumb, like the classic 'blonde chick' that is in just about every movie that HAS to be the one that causes everyone to get killed or something (no offense just sick and tired of people HAVING to be stupid in these kinds of movies).
I'm surprised to see that Night Of The Living Dead wasn't yet listed, and I'm going to have to put that up for my vote for the best zombie movie of all time. The original was groundbreaking at its time, and without it, much of our zombie-loving culture would not exist.
Outside of that, I have to give props to Dead Alive for just being a hilariously-awesome gorefest. It's funny to see that Peter Jackson made Dead Alive, and then years later went on to make Lord Of The Rings!
Evil Dead is also one of my favorites, just for its development of Ash as a standing point of pop-culture. That, and I just love the movie!
28 Days Later was one of the first "zombie" movies that really seemed to humanize the main characters and give them qualities that really made me root for them. I remember being on the edge of my seat during that film during Jim's struggle to find Serena. That, and I firmly believe that it's one of the most beautiful movies of all time -- some of the camerawork is absolutely breathtaking, and the soundtrack and filming fits together to make one of the most emotional experiences I've ever had wth a film.
The remake of Dawn Of The Dead was another groundbreaking movie to me. Although I love the original, the remake holds a special place in my heart as one of the best new-age horror movies. It took a ridiculous concept and made it serious, and it made it easy for me to enjoy the characters, like them, and hope for their safe escape!
Zombie, by Lucio Fulci ... -- I can't really say that it's an amazing film, but disgusting? Yes! That, and you have to give a bunch of credit to a movie that has a five-minute sequence of a zombie fighting a shark in the ocean.
Personally, I like my zombie movies to be fun. Do they need to be serious? No! Do they need to be bloody? Yes! Do they need to make me cheer for gore? Hell yeah!
Ok, I am Legend was awesome, but it didn't come to my mind because I don't really think of it as a zombie movie....dunno. I also forgot Night of the Living Dead, and even misspelled Shaun of the Dead! (Forgive me, it was early AM and I had to leave for work 5 minutes previous...)
A lot of people said that 28 Weeks Later was a pretty terrible sequel, but I actually thought it was really good, considering -- I thought it really captured the feel of the first one without trying to be too artsy-fartsy and too much like its predecessor to stand alone. Although I missed having Jim in the movie, I thought the cast of characters was surprisingly likable, and I thought that the twist at the end of the film was really quite good!