GTA for sure, but I am torn between GTA 3 and GTA: SA. I've spent HOURS in GTA: SA even after finishing the game. The world was ridiculously expanded, considered, when the game premiered and there were also mods. One of the very few games I've spent more time in after finishing the main campaign than on finishing the campaign.
On the other hand GTA 3 holds even more nostalgia for me than GTA: SA. It was one of the very first games I ever played on my own computer. And it was objectively a great game for its times, so it's not just nostalgia - it was a freaking good game.
Cannot choose between those two, both remind me of good times, both were great during their pride, both I've spent days in.
I spend more than 5.000 hours in this game and always play again after some time.
I started the WotL a while ago and I am loving it. The game is hard af tho. Definitely not a game I would ever recommend as a 1st time Final Fantasy experience due to how difficult it is. I had to take a break from the game and start another Final Fantasy in the meanwhile but I'm getting back to it maybe end of the week.
To add more to my list - World of Warcraft, Warcraft 2 and 3, Hearthstone, Heroes 3, Resident Evil (0, 1, 2 and remake, 3, 4, 7, Code Veronica, Outbreak), CoD Moderm Warfare (1, 2 and currently Warzone) and Half life series as a whole - still crossing fingers for HF3.