Well I haven't played it recently, but the worst game I ever played was Universal Combat. It's impressive in it's own right though. A lot of games suck, and some more so than this one. But this one is rather unique in just HOW MANY things it did wrong! Fly your carrier in space, do missions, do cool stuff in space, land, air, and sea! ...Well, actually the sea didn't exist. Just air combat with water beneath you. And the nearly-200 page instruction manual completely fails to explain how to play the game. And the "keyboard shortcuts" paper is three sections, front-and-back (yes, the whole keyboard is used, some keys multiple times depending on circumstance). The AI was stupid beyond belief. The game had you flying for 5 minutes for each foe you wanted dead, and was in desperate need of a time compression button. I could go on and on, but I won't. It's just amazing how many exciting feats it attempted and utterly failed at.
And it was the 5th (or something like that) in a series. My head hurts at that.