I wish I could have voted for two, because I would've gone for the VAMP and the MOBAT. I picked the VAMP because I figured most people would go for the MOBAT. Seems I was wrong about that.
The VAMP, in my opinion, showcased what G.I.Joe was all about -- MODERN Army action figures. The VAMP didn't look like a jeep out of World War II or M*A*S*H or anything, and yet it did look like a modern successor to the basic idea. And it's certainly been a vital part of the world of G.I.Joe ever since.
While the MOBAT is proportionately small for a tank, and was outsized by the Mauler and later vastly outsized by the Grizzly, I think it's important to remember that in 1982, no one knew if G.I.Joe was even going to work, which is doubtless why we got so many common body parts among the figures, and why the vehicles were somewhat limited.
Nevertheless, the MOBAT looks like a tank, the detail level on it is remarkable, and the method by which it turns on, goes forward or backward, and turns, based on the operation of the upper turret, is really very clever. I was pleased to see it brought back in the Target set.
P.S. - Like the idea of these vehicle polls!