I thought 1 had a lot or original ideas for it's enemies which I liked.
Although none of the DMC2 enemies stick in my mind that much now I didn't really think they were crappy, a lot of their ideas were still quite original like 1 but at the same time very hellish like the goat demons and I remember that fleshy thing you had to break down being a neat idea at the time.
It was DMC3 where I think they got lazy. It's like "hmm whats gothic? I know! the grim reaper!..and scythes and BATS...no wait! bats! MADE OF BLOOD!" I know they were just taking the design from the sins in DMC1 but...I felt the same way about those too and I felt taking it and expanding on it was the wrong way to take things. Anything in the series that looks like it has been thought up by an ageing death metal band ive never been a fan of and theres always something in every one.
I like the wierd and wonderful ideas like SRS said a shapeshifting shadow demon is a pretty creepy concept not knowing what he is going to become I remember when he first did that cage thing to me and I was like "what the hell!? He's turned himself into a prison!. (Edit:
I just realised that was nightmare not shadow duh)
All that sticks in my mind from 3 is silly gimmicks like "oh you can't shoot it yet, you can't attack it like that, you need to do this first". The others had dumb gimmicky demons too at times but for me in 3 nothing felt fun to fight.