For me it's Chrono Trigger. I always want to play it. I want to show it to my children. I hope it will be regarded as a masterpiece for generations to come.
If I ever make a game, it'll be a LotR2 remake. I have some ideas I'd like to play with these take it in a completely different direction from Total War games, such as:
play as the merchant
play as a general in the battles, while someone else handles empire management
play as a mercenary band
All of these could play together in MP, and each has a separate win condition.
But every time I get excited about building it, I replay the game and it scratches that itch.