Some games just have something special about them that I can't explain:
Shadow Blasters (one of the most mediocre Genesis games that I can't help but replaying from time to time).
Code Name Viper for the NES (It's a carbon copy of Rolling Thunder, but I like it about ten times better than RT); on the other hand, I love Rolling Thunder 2 for the Genesis with a passion, and don't like the third game that much.
Thunder Force 3 for the Genesis - I can't explain why it's so much better than all the other side-scrolling Shmups of the same time, but it is; maybe it's because it was my first Genesis game...
And of course, there is a game (or rather a series of games) which I could go on for hours and hours explaining why it's so much better than similar ones, but which still falls into the scope of this thread because not too many people share my opinion: Gothic (especially the second one). Wandering those beautiful landscapes and finding interesting things to do in every nook and cranny took away many hours of my life, and I still think it was not enough. Someday I'll have to post a review and shut up about that game already...