Child of the '80s. I honestly don't think I had one 12" growing up.
In university I couldn't help picking up an 82nd Airborne figure from Ultimate Soldier, but that was a one off (I still have it somewhere). My brother gave me the Sideshow Old Ben, which got me started on this little path... The first one I bought myself was the Exclusive Snake Eyes - man, I'm glad I started from the ground floor with the Sideshow GI Joe line!
12" Mego Batman also had all the Six Million Dollar Man figures. Modern times my first would be a custom Indy I made after seeing the price of the ToysMccoy one. First sideshow was the original Hush Buffy and the 12" universal Frankenstein's monster.