I was just browsing amazon.co.uk for some movies to put in my wishlist or maybe even shopping cart when I came across;

Diary Of The Dead - Limited Edition 2 Disc Steelbook Metal Packaging [2007]

This got some bad and good reviews (if you wish to comment on the above movie that would be nice aswell) and I read some comments which said things about other zombie movies (from George A. Romero).

That got me thinking since I know there are a few "must see" ones which I for sure never saw but no idea which ones .
I have seen 28 days and 28 weeks (which were good/great respectively) but further then that I don't think I have seen any.

So bottomline for anyone who didn't read the above;

Which zombie movies are must see movies and are actually good and not a failed effort to hitch on the hype.

I recently saw Diary of the Dead at the Brussels International Fantastic Film Festival. I didn't like it very much. It had a very cheap "shot for video" look, and the story wasn't that interesting.

Must See:
1968  Night of the Living Dead (not the 30th anniversary version with newly added footage)
1978  Dawn of the Dead
1986  Day of the Dead
1990  Night of the Living Dead (Remake)
2004  Dawn of the Dead (Remake)
2005  Land of the Dead

1979  Zombi 2 aka The Island of the Living Dead
1985  Return of the Living Dead
2003  Undead
2002  28 Days Later
2007  28 Weeks Later (still need to see that one myself)

And if you count the "Resident Evil" movies as zombie flicks, you can include those as well.

Return of the Living Dead has a few sequels I didn't like, but maybe you want to see them anyway.
There is 2006 remake of Night of the Living Dead in 3-D. It isn't any good but since you want to see zombie movies you have to see some crappy ones as well 

This should get you started.

Greetz,
Chris

Here are the zombie/undead/reanimated dead movies I really enjoyed. Though these are not really your typical Romero-type zombie movies.

BioZombie (chinese)
Choking Hazard (russian, I think)
Night of the Living Dorks (german)
City of Rott
Dead Alive (new zealand)
Undead (autralian)
Junk (japanese)
Shaun of the Dead (UK)
Wild Zero (Japanese)
Stacy (japanese)
Dead and Breadkfast

Also, everyone has their option on the 28 Days/28 Weeks movies, but I don't consider then zombie movies.

Dead Alive is a must see zombie movie.  As is Shaun of the Dead.  Two of my favorites.

Of course, the original Night of the Living Dead, and the original Dawn of the Dead are, to me, zombie canon.  I would watch those first, and then move on to the many other excellent suggestions here.

x  of the Dead are all the same "serie"? So I know in which order to watch them and also so I can buy them all then prob (as said by cvermeylen).
Is the story in any way connected? (not that it matters but just wondering)

I'm also asking this cause of all the bad movies there are (even more compared to other genres).

Thx people for all the suggestions so far.
Guess my shopping cart isn't full yet... breaching 400 pounds now 

I'm really trying to see all the "best movies ever".
Got a bunch of movies from the '40's and '50's. Bogart, Garland, Kelly and the like.

Now for some zombies .

As long as you remember most zombies movies are campy and have at least some funny parts to relieve tension, there really are few really BAD zombie flicks.  Al what definition are we using Hell is full or the infection paradigm?  Here are my favorites in no particular order some people will call them horrible but I will always adore them:

Dawn of the Dead (2004 Remake). 
I loved I think they did a good job reintroducing the genre to the younger generation)

GrubGirl (2005)

Night of the Comet (1984)

Resident Evil (2002)
Resident Evil Apocalypse (2004)

The Return of the Living Dead (1985)
The redhead makes this movie

Robert Rodriguez's Planet Terror (2007)

George A. Romero's Dawn of the Dead (1978)
George A. Romero's Day Of The Dead (1985)
George A. Romero's Diary of the Dead (2007)
George A. Romero's Night of the Living Dead (1968)
For this film make sure you purchase the Millennium Edition.  It has a far improved print which is not muddy like many others.  There is no new footage either which seems to have annoyed many.

Serpant and the Rainbow (1987)

Shawn of the Dead (2004)
I hated this film but everyone I know adored it so it must be good and I am just missing something.

Warning Sign (1985)

Zombie (1979)

I am not really a fan of "Land of the Dead", "Flight of the Living Dead",  or either of the 28's.
I hope this is a good start if I have missed anything please feel free to add it.

@JackKnight

You correct about being an acquired taste, but is true about a lot of Asian Cinema. I find it easier to follow if I use the native language and subtitles. IMHO.

Rory