For me, it's gotta be Minecraft Java Edition. In the early stages of Beta, I downloaded one of the cracked launchers way back when and started to fuck around with the game, and eventually after a couple of months, I ponied up the money to buy in when Beta 1.6 rolled around and I haven't looked back since.
I remember the days when mods were the exception, not the rule of law, and the biggest and boldest were ones like IndustrialCraft, where it was revolutionary to have pistons, where the terrain generation might just spawn a lava lake at the lowest bottom layer of bedrock and you could thus get blown into the Void by a stray Creeper in your strip mines because your dumb ass didn't calculate light levels right.
