The worst game I've played, to date, was ODT for the PSX. Never, in my life, have I been so disappointed that I begged to have the game returned to Blockbuster the same evening it was rented. It was upsettingly bad.
As for the best game, there are a few. Among them are Mega Man 3 (one of the only games I can still sit down and play through again and again without getting bored, plus it's the only one I actually find easy, for whatever reason). Contrary to what was said above, I did like "Another World" (though here, it was marketed as "Out of This World"); there's just something about it that I enjoyed - similarly, I liked Flashback, which was of the same style, but greatly improved. Neither were my favorite, but are worthy of mention, I suppose.
Also, Warcraft 2 remains great in my eyes. Easily one of the best games I've ever played.