Really is just self-explanatory but considering how this game is a bit easier than the first one I just want to know who gave you the most trouble however mine turned out to raspatil. Your turn.
Including DLC? Gilgamesh. Haven't fought LV99 Valfodr, so I can't compare the two. No DLC? Raspatil, mainly because of the constant mook summons and what it does with them, but still fairly easy if you know what you're doing. Some people might say Yomi, but its really just a heavily nerfed Vercingetorix from the first game.
Gilgamesh is really that difficult? I never really fought him. The only DLC I got were the ones which included Snow and Lightning(both of the Lightnings.)