I'm not sure if you guys have noticed, but I sometimes tend to go for sequels over original movies in franchises. It's because sometimes I prefer a sequel to the original (Friday Part VI > Friday the 13th).
Day of the Dead is a movie about the zombies taking over the world and a small portion of humanity living in a bunker.
I like Day of the Dead the most of the original trilogy because it doesn't have a message and it has really good effects AND a really good story. You can tell me until the cows come home that Day has a message but Romero was not a good enough writer to subtly slip in social commentary.
It is my personal belief that Romero and his friends cast Duane Jone just because he was a good actor. 1960s critics saw the movie as a social commentary of race at the time and Romero just went "yeah, that's me, Mr. Social Commentary." Then when he did Dawn of the Dead, he decided to slip in heavy-handed social commentary. That's not to say I hate Dawn, but it's what I prefer about Day.
I love how Day shows civilization has crumbled due to the zombie threat. I love the effects throughout the movie and they are some of the best in the entire series (taking into account Land, Diary, and Survival here). The zombies are gross looking and I love them.
I love Howard Sherman's performance as Bub. Bub brings a lot of heart to the film and is possibly the only time I've felt sympathetic towards a zombie (yes, I've seen Fido). Look, if you haven't seen Day of the Dead, you're definitely missing something wonderful.