If you meant by favorite to laugh at I'd have to say either the investment style ones, or the ones below
i think i have a problem, i'm addicted to infomercials....so i actually have a favorite that i prefer to watch at 4am...sad...yes..i know..
One time I had the flu and I feel asleep on the living room floor after taking some medicine and the TV was on. I woke up after having a dream about me and Snoop Dogg hanging out getting girls to flash us. That Girls Gone Wild infomerical with Snoop was on TV.
Any infomercial with Ron Jeremy in it you know is going to be better then most PrimeTime television. The hedgehog has so many funny ones.