Forcing myself to complete Never Alone.

I completed xenoblade chronicles X and my knowledge of japanese is next to zero.

^This, getting the "Brass Balls" achievement for Bioshock, completing Champion's Road in Super Mario 3D World, and getting 100% on both Metroid Prime 1 and 2!

Beating TzTok-Jad in Runescape and getting a fire cape.

Beating the highest ranked Halo Wars player in the world in the finals of a tournament with an underpowered character to win $250. (pro gamer as a kid haha)

Getting diamond in League of Legends.

I don't play any of those games anymore though.

Beating Bayonetta with all platinum trophies in all difficulties. It was a sweet hell.

getting the Platinum in Vanquish, those challenges men... they make the souls games look like a walk in the park.

also the Platinum for vanquish is just the Platinum logo which is quite fitting and cool to be honest

Mine is kind of silly, but I managed to beat BioShock Infinite on 1999 mode without using the vending machines once, which I was pretty proud of.

Betting Dark Souls the first time without using any guides at all...not knowing where to go or how some things work makes this game 1000x more difficult than what it is. 

Managed to (almost) reach one of the endings of Wizardry IV

If you think only almost reaching an ending ain't worth much, just check this (written) Let's Play and you'll see how infuriating difficult and cryptic this game is (probably even the most difficult game ever made): http://lparchive.org/Wizardry-IV/
I mean, after the first edition of the game every single copy came with a sealed letter with the solution of the very first puzzle - and that one is by far the easiest one in the game, as even the clues to the metric crapton of cryptic puzzles and riddles are cryptic riddles in themselves. And when I played it some 25 years ago, there was no internet to find a solution to the puzzles, I had to figure them out themselves

100% completion in Grand Theft Auto V + the platinum trophy.