Of course Lucas thinks ESB is the "worst" one. He's a control freak and it's the one he had the least control over.
Plinkett points it out very adeptly at the very end of his TPM review: when Lucas finally had absolute creative control and effectively unlimited funds and resources, he made the prequels. He wasn't struggling with a studio or forced to make any compromises. He made them that way on purpose.
It's shocking to everyone else, but the sad truth is: the prequels are what George Lucas thinks STAR WARS is supposed to be.
So if the prequels are what he's always wanted from STAR WARS, it's very little wonder that EMPIRE, being the exact opposite of the prequels in pretty much every facet, will strike George as being the "worst" of the bunch. It fails to accomplish what he thinks a STAR WARS movie should.
Which, of course, is exactly why it's great.
Also, I hate those Brain Bug cats. They're ugly and whoever first decided to selectively breed them that way is history's greatest monster.