Honestly, Terraria or Minecraft, but modded. They are such loved games that an entire subculture has created thousands of versions of a base game, and I love it.
Want to play an enhanced version if MC? Gotcha. Want to play a fucking super-techy-space-magic version of it? Got that too!. Same with Terraria, Thousands of new items, bosses, mechanics etc. all for you to find your niche.
The ending/realization of Assassin's Creed 1.
It was such a big mindfuck for my young mind.
Also the "Would you kindly" from Bioshock.