I'm not going to get into the debate of which one was better... the 3 of them were beyond amazing.
Jimmy Fallon is entertaining, but there's something about him when he does comedic skits involving MUSIC. Then, he's really freakin' fantastic. Examples: Whip My Hair, the Bob Dylan impressions and of course, History of Rap 1 & History of Rap 2.
Justin Timberlake and Jimmy have such an undeniable chemistry, and that's why these skits are so damn hilarious and... epic!
On a side note, why is it that Jimmy is only a good interviewer sometimes? I have him all figured out...
If he's friends with the interviewee or has some sort of relationship with them, it's great (See: Justin Timberlake or Amy Poehler).
If he just met them, the interview is awkward, a bit uncomfortable to watch and he keeps fake-laughing at everything they say. For you to get a better understanding of a fake laugh, this is what Urban Dictionary says:
A person can be caught putting on a fake laugh by him/her laughing too suddenly, and ending the laughter suddenly, usually with a straight face throughout it or at the end.(See: most of the guests of the show like this one).
Back to awesomeness... Here's History of Rap 3: