I was going to say Nemesis, but when you think about it, there's not really anything he can do.
Nemesis is fast, Wesker is faster.
Nemesis can spread a virus, Wesker is immune to it.
Nemesis can shoot rockets, Wesker can catch them (unless he's submerged in lava, that is).
Nemesis is a serious threat to any ordinary human, but to a superhuman, not so much.
I also don't think Haos is all that powerful. What exactly are its powers anyway? Because the only thing I remember it doing is flailing around hysterically and destroying platforms, which is how it got Nivanfield into trouble, and yes, once it gets a grip on someone like it did on Chris, that someone is in serious trouble, but Wesker wouldn't even allow it to catch him (the guy can dodge bullets after all), and if it still managed to do so, he would be strong enough to break free.
Wesker is probably the strongest enemy we've ever faced in RE and I think the only thing that can defeat him is - ironically - a human. Someone he doesn't take seriously, so his superiority complex will do him in. And that's exactly what happened eventually.