If I had to pick a favourite season (which is VERY hard to do, MWC is my all-time favourite show), I'd have to say Season 4, Kelly is looking really hot (although she was hot IMO until she cut her hair short at the start of Season 9), we see the last of Steve, the actors are firmly in their roles, and several of my favourite episodes occur this season, including:

Hot off the Grill
Tooth or Consequences
It's a Bundyful Life
Yard Sale
Who'll Stop The Rain
You Gotta Know When To Hold 'Em
Peggy Turns 300

NOTE: The second last option in the poll is meant to be Season 11.