Without a doubt, classic Star Trek. My brother and I discovered the show when I was about 13 and he was 8 or 9, and I'm not sure "obsession" is a strong enough word to describe our relationship with the show. We had plastic figures of the major characters, and we went through phases where we played with nothing else for weeks on end. We would have plot lines that lasted for days, we created a host of original characters that periodically joined them for adventures, and we created fantastic sets of the bridge, sickbay, the transporter room, and more. Sometimes setting up to play Star Trek took up almost our whole pre-lunch half-hour of playtime, and then we'd have to wait until we'd finished doing our schoolwork to get any real adventuring in.
Yes, I played with action figures at the age of 14. With my younger brother. Matter of fact, we played with them until I went to college, and when I'd come home on breaks, we'd play with them some more. Not just Star Trek, but Star Wars, X-Men, Batman, GI Joes, and several other worlds. Sometimes we did cross-overs between different universes. It was awesome.
Anyway, we got into Star Trek when my dad ran across one of the movies on TV or something, and my brother saw a slice of it and pretty much fell under Spock's spell. I'm not sure if it was Spock's intelligence or logic or what, but Johnnycake decided then and there that Spock was the coolest guy ever. Kind of instant hero-worship, as I recall. Me, I was less interested (I'm still not a big fan of the movies). But Johnnycake found this set of action figures and wanted to get them, only he didn't have the cash to buy the whole set.
So he convinced me to go halvsies by agreeing to let me be the captain (who, to my perpetual embarrassment, I initially thought was named Captain Curt) if he could be Spock. Ever power-hungry, I agreed. We bought the figures, then set about learning about their universe. Our library had six episodes of the show on VHS, all from season 3, and those were the basis of our understanding of the Star Trek world for several years. Well, those, plus a handful of episodes we taped off cable while at our grandparents' house, and a couple more I bought on VHS. Later, the same TV channel that introduced us to Combat! began showing Star Trek, and we got to know the show much better.
We ended up with Star Trek calendars, toy phasers and communicators, oodles of action figures, books about the show... it was probably the biggest obsession of my teen years. Yes, I loved it much more than Combat! at the time, though over the years my interest has waned somewhat. But I still enjoy the show and watch an ep every now and then. And I still have my action figures.
Captain Kirk is hands-down my favorite character. Dear, ingenious, rash, skirt-chasing, swashbuckling Jim Kirk.
If you want to know what I thought of the recent movie reboot of the series, here's my blog post on that topic.