Ni no Kuni is one of those games that didn't wow me at first. But it grew on me as I played, and by the end, I loved it. I wouldn't argue that it's flawless, but favorites don't have to be. It had a good story, gameplay that was both fun and complex enough to keep me interested 80 hours in, outstanding music, and visuals that earned the only 100% I've given a game in any category in the nearly 8 years I've been writing for RPGFan.
I love a good hack & slash (as evidenced by the #1 entry on this list) just like I love a fun, uncomplicated book. But sometimes, you need some literature in your life, and in the realm of gaming, Planescape: Torment is a title that deserves to be thought of as literature. How many other games even try to thoroughly explore a core question like "What can change the nature of a man?" It offers many possible answers, and the beauty is that you can decide for yourself what the best answer to the question is. It also wins a spot on my list for being one of the only games I know of where making the main character as wise as possible will give a better ending than giving him the biggest sword he can find.
I've happily played through this game at least three times, and when I heard a (likely untrue) rumor recently that it was being ported to 3DS, I knew I'd happily buy it and play it one more time on a new platform. It's fun and stylish, with a decent story, lots of humor, and perhaps my all-time favorite battle system. It's a turn-based system where you don't just sit and wait between choosing actions, but you still have time to think and strategize between turns. I can't recommend this one highly enough.
My colleagues recommended this game to me for years before I finally got to play Persona 4 Golden on Vita, but when I did, I finally understood what they loved about it so much. It's a masterpiece, and I'm not ashamed to say that I used a guide to make sure I maxed out every possible social link. I had to know more about those characters. Like many people, I've often felt that society pressures us into representing ourselves as something other than what we really want to be, and I really appreciated the characters' struggles to break out of that trap. Kanji and his love of sewing, Naoto and her skill at detective work (which girls aren't apparently supposed to be good at...), and even Yukiko, who fights against what she sees as her obligation to take over the family inn until she realizes that taking over the inn is actually what she wants to do. She just didn't want to be pushed into it. Everybody on the team goes through intense self scrutiny and comes out the other side with a better understanding of what they really want out of life and being at peace with their choice, and I love it.
There have been plenty of other hack & slash games I've enjoyed, but none has stuck with me like Diablo II. I played that game on and off for five years, and I still have a USB in my backpack with the game installed and all of my saves handy, 15 years after the game's initial release. It was great to begin with, and the Lord of Destruction expansion made it even better. Since I played the single-player campaign almost exclusively, I never did manage to get my beloved necromancer or my druid to level 99, but I had hundreds of hours of fun with them and the rest of the dozen or more characters I played over the years. There may be other games on my list that are objectively "better" games than Diablo II, but none of them will ever hold the place in my heart that it does.