Picking one is so hard, but if pressed it'd probably be 28 Days Later. The Danny Boyle/Alex Garland/Cillian Murphy trifecta worked so unbelievably well there, and the way they realized post outbreak London plus the English countryside bits made for one of the most immersive films I've seen, zombie related or not.
Its hard to beat the OG Dawn of the Dead for the classics, but I've grown partial to Messiah of Evil even if that's kinda more vampiric in nature but it has a lot of zombie trope vibes in how the "infected" are presented.
Cabal Return is a good shout, that holds up as one of the best under rewatches. Its just so fun with the perfect amount of camp and great casting.