On the Platinum difficulty scale
Sonic Unleashed - 9.3/10 - The dangerous combination of not only difficult, but also time consuming because of the difficulty. It's enough that you have to find all 200 sun medals and 200 moon medals hidden throughout the game (found in stages, hubs and by doing villager missions), but then there's the hot dog stand missions. There's 15 of them per world and require you to be able to play a stage perfectly and quickly on one life so you end up practicing a stage over and over again until you have it down. I'm sure I spent over 50 lives getting this stage down perfectly yesterday -
LBP - 6.5/10 - This is more time consuming than difficult. Getting all the prize bubbles can be tricky as well as setting up some of the online trophies like playing with 3 other people on your friend list. However there are still a few hard spots like acing a lot of stages in worlds 7 and 8
RE5 - 5.5/10 - This was my first platinum so I remember it well! The only one that's remotely difficult is beating the game on Professional, but since you can use all your upgraded weapons like like magnums and rocket launchers, along with infinite ammo. Again most of the rating comes more from the time it takes than it does the difficulty
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