Ninja Gaiden and it's not even close when it comes to the combat system and relentless enemy AI.
Ninja Gaiden black is still one of the best - if not the best - character action game and it was released almost 20 years ago.
Ninja Gaiden 2, while more linear, perfected the already stellar combat of the 1st game.
NG3 doesn't exist.
The reason I'll always pick NG over DMC is the fact that in NG, you're actually fighting enemies and trying to survive the on-screen onslaught - a stark contrast to DMC where...you're there (mostly) to look cool and try to do SSS combos and rack up those points, they're games with a different philosophy behind them but I prefer the NG one (the 1hit thing in HaH mode doesn't count).
The only thing I dislike about the NG games is the atrocious, gaudy, kitsch art-direction, my fookin' Lord it's bad - Pink Demon's, big titted hammer -wielding pornstars, worms, cockroaches, TMNT Krang-like enemies, general boss design etc etc, a problem which DMC never had since it was always the more aesthetically pleasing franchise due to the better art-direction, music etc.
Basically :
Ninja Gaiden : combat
DMC : everything else