I think the best hacks where they have improved on the originals tend to be the space invader type games. As good as Space Invaders is, it is even better with the original arcade sprites. Galaxian is a great game that had one issue which is resolved by removing the border. Finally, Megamania is even more fun shooting aliens rather than obscure objects.
Therefore, my top 5 hacks are:
1- Space Invaders Invasion by Scott Dayton
2- Galaxian ND
3 - Arcade Space Invaders by Franklin Cruz
4 - Intellivision Lives by Scott Dayton
5 - Ventrra Invaders by Charles Morgan
Unfortunately, I can only play these on Stella as there don't seem to be any PAL versions available. If the people that programmed these hacks or one of the other many talented people on here could help with this it would be greatly appreciated. If a PAL version of these could be made available in the store, that would also be great.
What are everyone else's favourite hacks??