It's been awhile back now, but I beat Red Dead Redemption with the Completionist achievement, which is beating the game 100%, every outfit, every gun etc etc. Truly a shining moment when you see that achievement pop up.
Also, I beat Ocarina of Time on my 3DS for the first time ever. I didn't have a N64 as a kid, I had a Saturn and my friends had the 64, so I never felt the need to have one. I had tried playing OoT on the Gamecube when they released the Special Edition disc that had the game on it, but the graphics were so far behind the then current titles, it was kinda distracting to play, so I didn't get very far at all. When I learned it was coming to 3DS I about lost it. A lot of my friends had given me shit for never having beaten "one of the greatest games of all time". After having finished it, I can truly say I agree 100% on all facets that OoT is one of the best games ever created. The graphical overhaul and addition of 3D were certainly an awesome addition to an already wonderful game.
Also, I beat Ghost Recon: Shadow Wars for 3DS a couple months ago, and that was a really good playthrough.