Joan Rivers -- Finally ... Someone Funny Is Hosting 'The Tonight Show'

Joan Rivers thinks Jimmy Fallon is the perfect replacement for Jay Leno for one key reason -- he's actually funny!!!

Rivers unloaded a barrage of Leno insults on our photog yesterday in New York City, while heaping praise on Fallon ... who she says she's been trading emails with since news of the "Tonight Show" switch broke.

Rivers' beef with Leno and the "Tonight Show" has been going on for over 20 years ... which coincidentally is the amount of time Rivers says Leno has been on the air ... and done nothing memorable.