Oh, K' rocks too, but I don't normally use him with Kyo cuz their attacks are similar, that is to say, they both fry people in flames. +__+ And they talk a lot. Noisy.
Testament!! Ahhhh. When I was new to GG I could never beat Testament, and everytime his Tension was at its max I would start cursing my head off. I personally think GG is more difficult than KoF - but KoF's more fun cuz you get to play with more than one character.
I also like SF vs Marvel Super Heroes - you can do character-combos (my fave is Sakura x Dark Sakura). But I haven't played that in yeeeeaaars. o.O;;