Here's a few that come to mind. More of a review than what they could do to improve. But as a general statement I'd say that both Marvel and DC have had a couple of outstanding villains, a few noteworthy ones, a bunch of serviceable but forgettable villains, and a handful of truly awful villains.
Marvel............
Magneto (yes, technically he's fox) : the best, he's had 6 film appearances and 2 brilliant actors. In fact he's almost an anti-hero, rather than a straight up villain. We can kind of sympathize with him, which makes him all the more relatable.
But, so far he's been the only worthwhile villain in any of the X-Films. Kevin Bacon doesn't even come close.
Loki: Close second, Hiddleston gets better with every performance. He was a real scene stealer in Thor TDW, definitely Marvel studios best villain.
Green Goblin: (from the Raimi film) pretty good. Willem Dafoe hamming it up, but convincing as a nutcase.
Robert Redford was a pretty decent evil megalomaniac in Winter Soldier, whereas the titlular character just hits stuff, not really a tour de force performance.
Obidiah Stane wasn't bad in Iron Man, but that film didn't need a nemesis type of villain, as it was all about Tony's change of heart.
Dr Octopus: A solid performance by Alfred Molina, probably the best we could expect from a live action Doc Ock. I'm not sure I would rate him even close to Loki, Magneto, Ledger's Joker or even Shannon's Zod - but he does the trick (despite the whole fusion reactor thing being really dumb - they chucked a mini-sun into the river, OFFS ! Come on guys that was beyond stupid, it's on a par with flying around the world really fast to turn back time.
Malekith, Whiplash, Red Skull, Sabretooth, Venom and Sandman: not great, very lacklustre performances.
The Mandarin...........ugh !
Dr. Doom doesn't even deserve a mention, simply dreadful.
DC
The Joker: probably the best comic book villain ever. Well served by good direction and two great performances. Leto is going to need to bring his A game to even be passable as this iconic character.
Sinestro, probably the only good thing about the GL movie, and Mark Strong at his villainous best.
General Zod: enjoyed both performances, Stamp and Shannon. Very different versions of the character but each worked well within the context of the film.
Lex Luthor: pretty good all around, I mean in the 70's Hackman's hammy performance was pretty standard. Looking forward to Eisenberg in 2016.
Two-Face: Great in TDK, awful in Batman Forever.
Riddler: no matter what anyone says, I loved Carrey as the Riddler, probably the best thing in Batman Forever.
Bane: Didn't really like Hardy's mumbling warlord version, a huge waste of his talents. Surprised Nolan got that so wrong.
Ra's Al Ghul: Pretty solid performance by Neeson.
but DC has some **** villains too, like the fart cloud in GL, the witch in Supergirl, everyone in Catwoman, Tommy Lee Jones as Two-Face, Uma Thurman as Poison Ivy, Arnie as Mr Freeze....
So all in all, I would say that DC has had as many crap villains as Marvel, but possibly a couple more really outstanding ones, and probably holds the heavy weight champion belt for evil with Ledger's Joker. I was really hoping that Ultron would be an outstanding villain, probably the biggest disappointment in Age of U - the real villain is Tony Stark's paranoia.