Gill was cake honestly. Seth is a lot harder, mainly cause he does actually have "cheap" moves. This also is a good thing so that not every person in the world is like, o ya, I beat SF4, I master, I rock, blah blah blah. I'd rather play fighting game that has a challenging single player that you actually get practice on then play only multiplayer cause the single player easy and boring.
With that said the only "boss" type character I fought in any game that was tougher then him was Broli in M.U.G.E.N. mainly for the same reasons.