well in one episode of them having their own friends they all starting missing him and all of the things he does to annoy them, however, it seems that has made him the center of the group instead.

Before Leonard Sheldon didn't have a friend. He brought Howard and Raj into the group and insisted on talking to Penny when she moved in. Without Leonard Sheldon would be an obnoxious friendless shut-in and Penny would be the girl who complains about her obnoxious friendless shut-in next door. Who knows where Howard and Raj would be.

I think it's Leonard! Of course they all missed Shelly(who wouldn't?), but Leonard is the one who can communicate with everyone and to hold them together. He's the medium.

It's gotta be Leonard, he's the one who originally all brought them together as a foursome. If Leonard wasn't there I don't think Howard and Raj would hang out with Sheldon as much to be honest, not that they aren't friends with Sheldon but atleast when Leonard's around he can get Sheldon to chill out when he's over doing it sometimes.

I think it´s Leonard, but Sheldon is my favorite character.. :D

What CelestialDream and detty4ever said.

Leonard is the reason that Raj and Howard put up with Sheldon haha, especially Howard!

I too think its Leonard who holds the group together

Yeah he really does. He was the one to introduce Raj and Howard to Sheldon, and he made the effort to get Penny integrated into the group, too. I think now the relationships could survive without him, but he's the entire reason they all started.

raj <3

Everything is about what Sheldon wants! He picks the restaurants, and whenever Leonard, Howard and Raj come up with a plan and Sheldon doesn't like it, they immediately give in!
So yeah I'd say Sheldon.

I'd have to say leonard, but Sheldon is my all-time favorite.