I had a HUGE M.A.S.K. collection as a kid. Still in the attic. I loved that series aside from the fact that no figure could hold a gun (nor did any come with weapons). That was an innovative toyline though.
My "most obscure and unfortunately overlooked" toy award goes to a line I collected for a long time called StarCom. Without a doubt it was one of the most innovative toys of the 1980's, the play value was outstanding and the toys were insanely complex in design and construction, even compared to the stuff we see these days.
The line's last items, mostly European-only releases, are incredibly tough to come by. Select U.S./Canada items are toughies too, or are tough to get in good shape, but the line's just outstanding and it even had a great cartoon that accompanied it. Very realistic, very expansive for its short run... Had me hooked, however I'm to the point I'd probably sell my collection if I found the right buyer just so that I could free up room around here.