I actually like Spider-Man 3 as much as the other Raimi films. I just don't particularly like Raimi's films in general (great villains, but I can't stand Tobey as Peter or Dunst as MJ...). That probably invalidates a lot of my opinion, but...
Anyway, yeah, I'm going with TASM2 on this one.
Rhino might have just been a cameo, but he did something I've been hoping they would do with Spidey films for years: introduce a smaller villain for Spidey to face at the beginning and end of the film. Just to show that there are other threats that they didn't need to cover on screen in Spidey's world. To show that it isn't just stopping generic villains and bank robbers all day. And, honestly, while not really developed, Rhino's scenes were fun.
Electro was a little weak in terms of narrative, but he made sense. He wanted to be recognized, had a chance to, and went berserk when he felt that Spider-Man was taking away his recognition. Which, to be fair, he was. It's not that weird, considering that Electro is clearly unstable from the get go. He's not like Doc Ock or GG, who were mentally stable and well in the head before they became bad guys. No, Electro is clearly nuts long before becoming a bad guy. I was surprised they took so much time justifying why he was a nut job. He seemed like the kind of guy who would come into work one day with a sawed off shotgun just to prove he was there.
As for DeHaan...I still don't get why people think he was underdeveloped. Aside from the fact that the Goblin syrum clearly screwed with his brains (he only laughs like a maniac while transformed), he had very good reason for hating Spider-Man and Peter. The guy didn't jump at the chance to save his life. He would have died. I fail to see how that's a bad motivation for a character's introduction.
And yes, it's an introduction. TASM series appears to be a story that is better told when you have all the pieces, unlike Raimi's films, which, individually, were complete and self-contained (save for Harry's arc). Looking at it like that makes the ASM series a better narrative. A little messy, but it works better that way.
Looking at SM3...it's a mess.
Look at Harry. Becomes Goblin at the start, builds up to a huge conflict---amnesia! Taken out before things can get going, until the plot needs him to do stuff. And what does he do? Force MJ to break up with Peter. It's a dilemma that could be solved if MJ just TOLD Peter that Harry was the Goblin. Sure, James Franco is just awesome as Harry, so that makes it easier to overlook the flaws...but what was Harry's plan? Just to screw with Peter? Okay, fine, but that's not really villainous. It's a little *****y, but that's...not really worth the three films of build-up.
Look at Sandman. While he had some great scenes, and his motivation made sense, Raimi had to retcon major details of Spidey's history just to add that personal element that was so essential to the plot. Complain about TASM all you like, but the fact that only half of the villains have a personal tie to Peter before becoming villains is a huge plus. It avoids repeating the whole "this guy is a personal enemy of mine" thing, reserving it for only the biggest bads. If every villain has a personal connection to Peter, then what's so special about that?
And then Venom. A huge disappointment. It's kinda...sad. Complain about Electro or Rhino, but neither of them had a whole deal of tragedy or nuance to their origins like Venom. Electro and Rhino are both jerks, and become bigger jerks with superpowers. Venom starts off as a fairly decent guy (albeit a little unethical with journalism), gets screwed, blames Spider-Man (who, to be fair, isn't an unreasonable claim), then merges with an equally rejected alien. If told right, it could've been on par with Two-Face's fall from grace in the Dark Knight. Instead...it's a joke.
So yeah, TASM2 wins for me. Easily.