I'd have to say I'm proud of all my Platinums. But I think MAG is the most important; the one that I put a lot of time and effort into, and much more even after I platted it. I got the plat relatively early (compared to the time it takes to get it), and I'm sure I was one of the first 100 to plat, maybe even one of the first 50 to do it (yeah, after the medal nerf, but still. Got the ribbons done before the patch so ). Then I kept it at 100% after the DLCs were released, something a lot of people had a ton of difficulty doing (Road Runner anyone? )
But I have 17 plats; Assassin's Creed 2. BF:BC2, CoD: MW2, Darksiders, Dark Souls, Dead Nation, Demon's Souls, God of War, God of War 2, Infamous, Infamous 2, Killzone 3, MAG, Resistance 3, Skyrim (most recent), and SOCOM4.
All games I had fun playing and decided to go after the plat. Have a couple of others sitting with one or two trophies missing, but those are games that I don't have any interest in going back and platting; no point if you don't enjoy it, right?
I'm proud of achieving the 78th element on all the games in which I have. God of War series, Uncharted series among others.
But let it be known no other Pt stand higher than Demon's Souls. Kratos, ask the Slayer of Demons for his Blessin.
I have 19 and could have easily had more if I hadn't gotten bored of some of the games. Not to mention glitches that make trophies not pop. But I am most proud of my Mass Effect 2 plat. Insanity difficulty is a very good name for that level. What kind of robot doesn't die when you hit it in the face with a nuke?
Fallout 3, New Vegas, Uncharted 2, and both inFamous titles all make me smile when I see them too. I have three LEGO plats as well that some people laugh at. But they're really fun games and the trophies aren't always that easy.
Edit: Hmm, just noticed I haven't updated my portable ID in a while. I really do have 19.
My favorite Plat is probably Resistance 2, mainly because I got it so quickly.
Killzone 2 gets an honorable mention just because of how difficult it was to achieve. x_x
Fallout 3. It isn't the hardest platinum to get but the Fallout franchise will always hold a special place in my heart. It made me very proud to have a platinum in a game I loved every second of playing.
I only have four, mostly because I don't own many PS3 games.
God of War 3
By far, FFXIII. Not only was it difficult and long, it was also my first platinum ever.
Pretty soon I'll have Red Dead Redemption and Dead Space too as I am almost done with those trophies.
I sort of gave up on GT5. Just because of the endurance races :/ . But I heard there was an update with a save option during races so I might end up getting that one as well after I get Ucharted 3 of course.
(Part of me really wants the much coveted Demon's Souls trophy, but I'd need to be able to pass the first part of the game without dying to even try to attempt to get a trophy, let alone platinum XDD)