Friday the 13th parts 3, 4, 5 and 6. I've never been a big fan of the first Friday flick (with the exception of Savini's FX) or even the series overall, but I think these four middle films are the best the series has to offer. It's not saying much, but still...
A Nightmare on Elm Street 2 and 3 -- I don't think they're BETTER than the original, but I do think they're equally as good in their own ways, which in itself is rare.
Hostel 2 -- not a great flick but still much, much better than the annoyingly pandering first one. I liked the female characters better here more than the obnoxious and homophobic twits in the first film, and I enjoyed the glimpses of the bidding process in action. I also liked the reversals of the two male characters, and I enjoyed how the main girl finnagles her way to freedom. It's still a mean-spirited and nasty film, but from a story and character standpoint it's FAR better than the first Hostel, IMO.
Dawn of the Dead '78; like Garden, I love Night of the Living Dead and still think it's one of the greatest and most important horror flicks of all time, but I also think Dawn out-did it in every way.
Hellbound: Hellraiser 2 -- As much as I dig the first Hellraiser, I think part 2 exceeds it. It looks and feels grander in scope, the FX are more frequent and better executed and the story is more interesting and involving for me. It looks and feels like a waking nightmare. I don't like the Cenobite Doctor at all (one too many corny one-liners for my taste) but overall I just think it's a better film, both technically and imaginatively.
Evil Dead 2 and Army of Darkness: like I said with the Elm Street flicks, I don't think ED2 and AOD are BETTER than the first Evil Dead, I just think they're equally as good in their own regards.
Can't really think of any others right now but if any come to me, I'll be back.