Having enjoyed both, I'm going with Avengers. This surprises me, as I'm emphatically NOT a comic book fan (not that I DISLIKE them as such, just wasn't really something I paid much attention to), and I've frequently been annoyed by the sheer power discrepency of the team (two nigh unstoppable forces don't seem well suited to be on the same team as two powerless humans. Iron Man makes up for it by having hypertech and my sheer enjoyment of RDJ's portrayal and Captain America gets a pass by being augmented, even if I don't really enjoy him as a character much. But the two assasins just...don't seem to fit. The movie does a good job in making them (or at least one of them) seem important, but who would you really rather have on your side? A god of thunder, or someone with a few trick arrows?
Sorry, tangent. Point is, despite the weird tier setup, Avengers was awesome. Good dialogue, decent story, interesting references (most of which I missed I'm sure, but the ones I got made me smile), and just plain well done. Pacific Rim, on the other hand...I'm not sure exactly why I put it as lesser. Maybe it's my suspension of disbelief. There were few things tossed in there that my brain forces me to argue against. So does the Avengers, but logic went out the window their when magic and gamma radiation that doesn't kill you gets involved, so it's easier to stomach. It's the little things, too. Like using a ship as a club without it tearing itself in half before you finished swinging. Or the staggering idiocy of the "let's build a wall and hope that keeps them out" bit.
Again, I liked both, but going to go with Avengers I think.