Definitely Friends. I was a 90s kid, and watched it frequently on the television. It seemed to be the perfect sitcom, but as we gamers know: nostalgia is king. It probably is just as mediocre as HIMYM, but it has a special spot in my heart. Most people I meet can tell me their favorite episode, too, so it is a great conversational piece.
LOST. Almost too easy. Never in my life have I found a show that kept me engaged from week to week. The little subtleties like religion, science, faith, philosphy, etc. made for a unique show. On top of that, the whole mysterious plot around the island. I almost felt as if I was stranded on there with them. I've had a 1,000 questions week to week. Great, great show.
I grew up watching Friends all through school, we'd often have sleep overs and stay up until 2am watching nothing but Friends haha. I recently bought the special edition box set, it makes me really miss the old days when it was new. I'm really hoping they will go through with this reunion movie!