The two handed Viking (with the axe) is really strong. I can't say if he is the strongest, but he is good. His dash + Normal attack is one of the hardest attack to counter. It's very fast and has great reach that makes it hard for others to do anything but block. If they block, it doesn't really do anything, so you can dash away and try again.
The Samurai (Two handed dude) is also strong and annoying because of the speed of his attack and the damage it actually does. His heavy burning attack is annoying too.