The original Tonight Show host was Steve Allen. He always made me nervous. I was just a kid and my parents loved him so I watched it with them when they'd let me stay up.
When Steve left a nobody from nowhere named Jack Parr took over. It took me about 15 minutes to wish I had Steve back. Jack was a crybaby and had no humor and no personality. He was a world class wimp. The night he said the words "water closet" on the air and the network cut him off he quit with big tears in his eyes. He was pathetic.
Then we got Johnny Carson. He was a class act the like of which we are not likely to see again. I kind of got used to Jay Leno...a little bit. He had a few good features on his show but he isn't a patch compared to Carson.
Carson had the best guests and the best gags and the most humor and the most class.
Letterman is a dork. his humor is stupid (Will it float?

) and As far as O'Brien if it wasn't for the stupid haircut he'd be totally forgetable and Kimmel IS totally forgetable. I may not be the best judge of anything but I do know what's funny and what's not and these guys are NOT.
I will always remember the last Carson show. One of his good friends, Bette Middler, sang a song to him, "Gimme One Drink For My Baby and One More For The Road...." The friendship and affection between those two was memorable