DMC3 SE and DMC4.
Best DMC games I've ever played. Also the only ones I've ever played.
Do I need to actually list why they're the best?
...
Okay!
DMC3:
-TWINS!!!!! (ahem...pardon me)
-Great story
-Random lulzwtf moments (giant flying whale comes to mind)
-Vergil
-
Dante Topless Dante
-Beowulf
-Dark and gritty gothic setting
-JACKPOT!!! <3
-And it's got a lot of awesomesauce in it, on it, whatever. It's awesome. It needs no explanation. Awesome is awesome. Deal with it. ^_^
DMC4:
-Better graphics!
-The cutscenes (are to die for)
-There's a girl! And she's not flaunting her wears! HURRAH for decency!
-The love story <3 Cliché, but who cares?
-Nero

-The Devil Bringer (hell yeah)
-Red Queen
-Rebellion
-Yamato (I love me swords....shurrup)
-...Dante
-Creepy background music
-Giant toads
-Different styles on the go (especially abused Trickster, Swordmaster and Darkslayer)
-Save the girl! while the credits roll
-Kick Dante's butt

-Angelo Credo (is awesome to beat)
-Bloody Palace - LOVE playing through it with both Nero and Dante
-Dante gets a kick in the face - BOOJA! I can't get over that. It was an awesome kick. Well done, Nemo.

-The ranks, wayyyy cooler than DMC3's
-The higher levels - gimme moar demons, moar demons, moar!!!!