From what I've seen of The Avengers so far in trailers and clips, it looks like their greatest nemesis will be Thor's boring brother, Loki (Tom Hiddleston). Admittedly I haven't seen the film yet, so I'm not sure what Loki's abilities look like or how much of an opponent he will be for the team of superheroes.
That being said, Batman's nemesis in The Dark Knight Rises is Bane and the film's trailers - especially the awe-inspiring latest one - have focused on this evil terrorist. From blowing up a football field full of players, to bringing Batman to his knees, this monster could be the greatest threat that Gotham has ever faced. And Tom Hardy, who is quickly establishing himself as an A-lister, seems to be great for the role.
Let's remember that Batman's previous enemy The Joker was portrayed by Heath Ledger, who won a posthumous Academy Award for his unforgettable performance. Bane might not match up to that, but I think Hardy will create a great and memorable adversary. Case in point: people have already started parroting their favorite Bane quotes.