Harold J. Lee on September 2, 2010 ("Deniro and Pesci eating latenight food from Pesci's mom in Goodfellas with the body in the car trunk. #UnsettlingEatingByPowerfulMenInMovies").
By the way, if Lee's name sounds familiar, it's because Harold & Kumar creators Jon Hurwitz and Hayden Schlossberg named the John Cho character after their friend Lee.
WARNING: NSFW language ahead.
1. De Niro's loud sandwich-chewing in Cop Land
2. Bernie Mac's loud orange-chewing in Bad Santa
Right at the 2:00 mark:
3. Kali Rocha, as a pregnant and surly receptionist for a city councilwoman, attacking a hot dog filled with sauerkraut in the Monk episode "Mr. Monk Fights City Hall"
Robert Wagner as a "Sloppy Eater" on SNL in 1989 (starts at 06:12)
5. Paul Sorvino's oyster-slurping in Dick Tracy
At the 0:28 mark: