The first one that really grabbed me was a 1911. Before that was a Crossman air rifle and then a Glenfield Marlin 25. The 25 was my first real gun and I still have it, but it was the 1911 that really got me hooked. I don't know if it counts, but I had a cheap, spring action 1911 BB gun when I was a kid that probably started it all. It was mostly a toy though.
That 1911 BB gun would be the ubiquitous Marksman, which (useless trivia, but still interesting) is the exact same BB gun that Chevy Chase use to take John Candy under gunpoint to ride rides in the closed Wally World in National Lampoon's Vacation.