I'd have to say unlocking and beating Ace Mode in RL, because I have absolutely no idea how to do it.(I guess get all golds?) I haven't even gotten silvers in all the levels, or maybe I have...I'm not sure. But getting any gold is so insanely hard...I never even felt like trying. SMS I did not think was THAT hard, I got all 120 shines. The only slightly hard thing was getting all the blue coins...but I liked searching every part of each level to find them. And Master Mode in SMB 1 and 2...maybe I'll go back one day and try to unlock it and beat it in SMB, I don't know about SMB2, I just don't like the levels as much. Getting all golds in all the challenges and arcade leagues in Timesplitters 2 is pretty hard too. But I've actually gotten a few platinums. I haven't even started story mode yet. I think the challenges and arcade leagues are just pure fun, but those pare in comparison to the multiplayer, which is unbelievably fun.