Pokémon, obviously. I can't even count how many hours I have spent in my entire life playing Pokémon, honestly. I play both casually and competitively, generally perfecting teams and the like after the story mode has concluded. I also love to mess around with all the different features in the game, and I tend to be damn good at all of them. I've owned one game from every single generation of Pokémon at the very least, if not all of them at some point in the past. I've probably caught them all, though spread out over time, even if battling and amenities are the main focuses of my playthroughs. (Pokémon amie, contests, secret bases, maxing out my teams, breeding for excellence, or even teaching new players about all those things.) I can name them all by cry alone, even those from Generation VI. I just want them to bring back a proper console based Pokémon game like Gale of Darkness. No more Battle Revolutions, please.
Another game I have dedicated myself to is, funnily enough, only two years in the making. War Thunder. I love the history in the game, and I read up on all the vehicles (though mostly the planes) and the battles they took part in and the aces that flew them. The Mighty Jingles is a great source for this kind of information on Youtube. I learn plenty about design models, combat tactics (both basic and advanced), the countries that used them and the various types of ammunition they carry. So it's a mix of educational and competitive, and I really do love when a plan comes together in one of the various game modes. Hell, I have a better K/D ratio in it than I did with Halo, and I am a monster in Halo. That K/D Ratio is around 5.5 or so, which is pretty crazy considering how I started out.