Tough call for me as I have a lot of vehicles that I have pride in owning - my Action Force RAM, APC, Panther Jeep, Laser Exterminator, Escape Armour, etc. those are a lot of fun, as are my Sears exclusive CAT and SMS.
Y'know...this'll sound stupid but Toxolab. Seriously, that is without a doubt, hands down, my most favorite item in all of G.I.Joe playsets and vehicles. As a kid I had a big fiberglass in ground swimming pool and I put that Toxolab on a piece of industrial foam stock and built this little floating chemical weapons lab with left-overs from my then beat up Terror Drome and other vehicle bits and pieces and accessories. It was by no means impressive by my adult standards but as a kid, I loved that thing. Said Toxolab met an untimely demise as a missile testing facility one 4th of July in 1994...but as a collector, one of the first items I bought and still cherish to this day is my mint condition, complete Toxolab. It sits in my glass display case, all decked out with Sludge and Toxo vipers and some v2 BATs
