Truth be told, it's a fool's errand to try to name the best Python sketch since most are brilliant. My favorites include:
The Argument Clinic
International Philosophy (always gets me right in the humerus)
Dead Parrot
The Twin Peaks of Kilimanjaro
Olympic Hide & Seek
The Killer Joke (could make a good case for this one being the best)
The Furniture Store
Twit of the Year
Ministry of Silly Walks
Scott of the Antarctic
The Spanish Inquisition
Election Returns
Wink Wink, Nudge Nudge...an endless list, really.
Python were, without a doubt, the funniest ever, with SCTV (Canada) coming in a close second (pun intended). There have been more than a few very funny Americans, but nothing really to compare to Python and SCTV.