My personal favorite is Shaun of the Dead, and I think its also the one Ive seen the most as well. Not sure if its a proper answer because of its satirical tone.
On the more serious side, Train to Busan and 28 days later would probably be amongst the ones Id consider "the best". 28 weeks later is good, tho not as good as the original. And of the more “classic” ones, the original Dawn of the Dead (the remake is also good), Night of the Living Dead and Re-Animator would be amongst my favorites.
I think the first REC was good but the zombie theme went to shit with the horrible sequels in favor of demonic possessions, which sucked.
Unpopular opinion: I never cared much for the original The Crazies (tho I like the remake), army of the dead, day of the dead, and such and such cause I think they weren’t that good to begin with and even aged poorly.