Metal Gear Solid Peace Walker. I have replayed all the Main Ops and Extra Ops and S ranked them all with my best friend to get ALL the Design Specs for the weapons and items. I have them all, but I still haven't managed to get the usages of all weapons to Level 3. I was a bit close to get the FOXHOUND rank, but I made a mistake halfway, and decided not to keep trying to get it. Still, I got all of the mechs, I found all of the alternate parts for Zeke and I got all of the memory boards, which makes him a beast. And the Monster Hunter missions... damn them. Tigrex specifically. He's even more difficult to beat than Gear Rex.
My friend and I spent many days doing all the mech Extra Ops, whether it was to capture mechs by S ranking the mission, or just blowing the hell out of everyone with RPG's, the Raligun while one of us charged it with the Dynamo, CO-OP Thunder, or any other hilarious way to take care of them. We have tried every possible mission in every outfit:
Naked with camo pants for that extra challenge
Neo Moss camo for mech battles
Sneaking Suit was my favorite of them all. It was my default outfit. Used it almost all the time for mech captures.
Battle Armor for those AI mech battles. Chrysalis Custom was a pain in the ass.
Tuxedo, just for laughs.
Swim Trunks for more laughs.
I have also spent a lot of time recruiting S ranked soldiers to use on the missions and Versus Ops. Thanks to it, I know a lot of dialogue lines from memory, especially Big Boss' raspy "You're fired!". I knew who was S ranked just from fighting them. I have even found an exploit to keep doing recruitment for a while. I got obsessed finding a soldier with a certain face in S rank, but it was IMPOSSIBLE! Not even my other PSN friends had him in S rank.
If you have him in S rank or near S rank, please contact me 
While I suck at every other multiplayer, I beasted at Versus Ops. Sure, it's like a kid's version of MGO, but it was damn fun and I won a lot of times there. Sure, there was a lot more people who was better than me, but I was also good, and that was enough for me. I always used the sneaking suit because it was tactically superior to the loud BDU's.
In the end, I platinumed the game before I really had fun with it. Once, I deleted a 200 hours saved game while on a blind rage. I repeated the game and I currently have 300-400 hours. So I could say with security I have mastered this game.
