Favorite hero on duel servers.

I agree, Marbas is great against many duel-based players, and his resistance is very helpful. :)

Yes, it's often difficult for me to find a suitable opponent for him. Perhaps good opponents for him aren't among the Uber heroes – Vitya, Ravenna. Otherwise, Marbas is quite a formidable hero.
My favorite hero in Heroes is the one who is versatile enough to compete with any hero. Jalib is the only one who fits this role. He has won against everyone, including Ossir, Airi, Synita, Deleb, Nimues, and Rava. The only one he can't beat is Anwen, no matter what, even though she is weaker than them. He's just a bit unpredictable.
But my favorite on the server is definitely Ossir. Since version 1.5, everyone has loved him, and I love breaking Ossir with Vitya, Galib, Axel, Hagesha, and Ibby. :D