One PC-engine game that I never get tired of is The New Adventure Island. Been playing it almost daily again. It has that great pick up and play quality to it, like having an arcade game at home. Its not that common with platforming games. I like all the Wonder Boy/Adventure Island games (and have beaten pretty much all of them) but this one just might be my favorite.
Its so simple and fast that it just begs to be played again and again. I usually pop it in quickly and see how far I can make with a single life. Feels almost like playing a good shooter but within the platforming genre, it has the same flow in it.