Xenogears, FFVII, ME2, AC2, UC2 Or Xenoblade Chronicles... Such a hard decision but if I could choose only one it would be FFVII.
It's the game that got my Tekken and Crash Bandicoot obsessed young teen self into rpgs and a new genre of games, after VII came FFVIII Xenogears Vagrant Story and many more.
Xenoblade Chronicles is a close second I was dying to play an epic long ass rpg and I hadn't played one since Lost Odyssey which wasn't as good as I had hoped, I was also off work post surgery so it gave me something to do for hours and hours.
It restored my love for rpg games and I can't wait to play the remake.