For me it was my GI Joes hands down.
I am 39 (soon to be 40) so I mean the full-size GI Joes that were about 12 inches high. (they seemed huge when I was a kid).
I started out with the GI Joes that had the plastic molded hair and positioned hands. Then of course came Joe with "life like hair" and then the infamous "kung-fu grip".
Between my friends and I we had all the optional goodies too. I had the helicopter and the boat (complete with alligator and octupus - depending on whether or not you were playing that Joe was boating on fresh or salt water). My friends had the 6 wheel ATV, the jeep etc. We had the various uniforms, civies, guns, knives, hand grenades, helmets, tons of boots and shoes...you name it.
Winter was the best time of year of all. After plowing our street the plow-truck would make this enormous pile of snow at the end of the block. We would bring shovles and carve forts and caves in the snow and play GI Joe in Alaska, the South Pole, exploring the Ukraine in search of Russian spys and Russian Barbie Dolls (that is another topic all to itself)...etc. Usually Joe was closer to James Bond than a soldier. A spy is so much more fun than just a grunt.
My other favorite toy/s is one year (i think I was 9 or 10) my dad bought me my own set of tools, a tool box and a tool belt. I was in heaven. I would wear that toolbelt around everywhere I went. I looked like a 9 year old verson of Sneider from "One Day at a Time".
Other than that, I was happy with carboard boxes and rolls of duct tape. My parents still tease me that I was more happy with the empty boxes than the toys themselves.