From those games that I went through but halted at the end due to difficulty, loss of interest or not knowing how to beat the last mission or boss:
- GTA IV (Xbox 360)
- Valis (PCE)
- MGS 3 (PS2)
Almost finished:
Pokemon Gold, I had 250 Pokemon collected (only missing Mew), 2 teams of level 100 Pokemon trained by hand (no cheats or rare candies involved). haven't played for a decade (only checking the game every other year for half an hour or so) and then discovered my save file was lost in 2014. Duh.
Should have made an SRAM backup using the Retrode.