I totally agree on Tron. That game was a turd even back then.

My vote goes to Sinistar. It's a classic, and it's absolutely overrated. The only thing good about that game is it's atmosphere with nice sound & graphics. But the gameplay (i.e. pushing a stick around, mashing the fire button and hoping you don't get shot) just fell short in comparison to Defender, Joust, & Robotron.

It was the first Williams-developed game that you couldn't really get much better at with practice.