Master Chief Collection Legendary run, Solo. Holy hell, playing Halo 1,2,3,and 4 in a row on Legendary was insane O_o Not only do enemies do more damage and there are more of them, they actually get smarter and use some tactics against you. It was insane, and it basically ended up being like the movie Live Die Repeat XP But hey, got to say it was worth it
Second hardest would probably be Star Wars: Empire at War on hardest difficulty. I had to blow up THREE Death Stars, and spend half of my time wondering were the hec the Empire was getting all these resources from !