Gosh, there's so many. James Pond II: Codename Robocod (Amiga) was always a favorite that I could pick up and play anytime, but I think I have played that one out. Trolls I always found to be superb, even though I think the general consensus probably disagrees with me. I'll think more on this.
Super Mario Bros. (NES) was also a game that I spent entirely too much time on. Regardless of how many times I beat it, I still would fire the cartridge up and see if I could beat it better.