I have been thinking about this for a while. Most of the posts on this site are specific to the current project at hand, or questions from new people like myself who need the occasional help. Why not create a topic that branches from the norm, and see what makes the games you hack so much fun.
1. Final Fantasy
What makes this game fun to pick apart is all about the numbers. Its my favorite RPG of all time, and I am having a blast with the data searching. Putting in different values for stats, hit points, prices, attack parameters, and raising caps, are just a few things that make me love this game even more.
2. Ghosts n Goblins
This is one of those games that I play several times a year, every year. I am currently able to decrease the difficulty of the original by restructuring around enemies and spawn spots. The best part though, is using Tile Layer Pro to bring the graphics close to par with the two sequels in the series.
3. Legend of Zelda
I have yet to put any real effort into this one yet, but this will be just as fun as Final Fantasy for reasons of its own, when I fully get to it.