After over 115 episodes, jumping into the Top Gear "canon" could be a little daunting. To give the uninitiated a jumping-off point, I've compiled a seasons worth (8) of my favorite episodes.
Season 7, Episode 5: Jeremy, driving the fastest car in the world (the Bugatti Veyron), is ptited against Hammond and James in a Cessna in a race to see who can get a truffle from the South of France to the top of the Nat West Tower in London the fastest. One of the quintessential TG races, this episode give a great look at the "unique" way that the hosts challenge cars.
Season 8, Episode 3: The boys try to turn ordinary cars into amphibious monstrosities. In a challenge that the show would later reprise, you get a great feel for the presenters personalities.
(It's not looking good for Hammond)
Season 9, Episode 6: In a similar vein to the amphibious car challenge, the crew takes ordinary European cars and turns them into stretch limos. Even for TG, the results are pretty outrageous.
The Polar Special: Of all the big season defining specials (Botswana, Vietnam, South Africa) this is my favorite. You can find a HD version of this one that looks plain magnificent.
(Gin and Tonics for all!)
Season 10, Episode 5: In an extremely practical challenge, TG tries to find the best mode of transportation to get across London.
Season 11, Episode 6: Season ending challenge involving Germans vs. Brits. TG constantly reminds us of "the War" to underscore their quintessentially British narrative. Jay Kay tries to be the fastest Star in a Reasonably Priced Car.
(.... before zee Germans get here)
Season 12, Episode 6: No massive challenges or races in this one (Although there is an interesting look at "Communistical" cars). "Traditional" looks at cars like the Caterham are in store; along with a "complete" test of the new Ford Fiesta. The Mayor of London (Boris Johnson) is the SiaRPC.
(Clarkson turns the little Fiesta into a beach assault craft)
Season 14, Episode 1: There's an awe-inspiring GT trip to Romania, complete with Borat jokes (even thought Borat is from Kazakhstan.) Eric Bana is a great guest in the Reasonably priced car.