Lore wise? Probably Metal Gear, I love every stupid little soap opera twist. If the internet reception was anything to go by, I feel like I am in the minority of enjoying the MG story more as the games go on. People thought 4, the portable games, and 5 were the worst but those are my favorites.
If it was gameplay, probably not Metal Gear. I'm above avg at the games but not a top player, there's quite a lot of MG tricks and skills that I've never bothered with.
To pick a franchise where I am most familiar with gameplay mechanics, games where I can babble on and on about invuln frames or combos or cutting off tails, I would have to go with:
Guilty Gear (fighting genre), Monster Hunter (action-adventure) or Dodonpachi (shmup genre), or maybe some puzzle franchise like Puyo Puyo or Magical Drop. Some other one-offs would be Minecraft, Mount& Blade, Total Annihilation, Baldur's Gate 2, Super Mario 3 (but not Mario overall), and Gunz The Duel, based on the same criteria of many hours + many high level skills learned.
I've sunken a lot of time not only playing those games but also watching video clips, learning from articles or blog posts, and practicing various "high level moves" (lol). Watching tournaments / live events, too. And actually trying to "learn" the game at a higher skill level. Watching so-called pro players and imitating their skills is a pretty extreme level of dedication to a franchise, in my opinion.
