As the titles states, of which Platinum trophies are you the proudest? This could be from very hard to obtain ones to your favorite game/series etc.
Mine are, in no particular order:
Assassin's Creed II
The Last Of Us. (Even though I don't have it yet, the only trophies I still need are the collectible ones and upgrade all weapons. So I will have it soon.)
(I hope I will be able to add Uncharted 3 to this list also.)
What are yours?
The Platinum Trophy that I'm most proud of obtaining is for ICO. That game was a ton of fun plus you have to beat the game in under two hours which is not an easy feat, my playtime ended up being 1:58:34. I beat the game probably 4 times in one day just to get that trophy and got all of the other trophies along with it.
Uncharted 1 was my first PlayStation 3 Platinum. What I'm most proud of though is my Vita trophies, again, I'm most proud of Uncharted Golden Abyss being my first Vita Platinum.