9x12 The One with Phoebe's Rats [5th or so watch]
9x13 The One Where Monica Sings [5th or so watch]
I swear they used clips from other episodes in the opening montage, but this is a pilot with a six-episode series set to follow later in the year. Which makes me happy because it was really rather good.
[Watch it (again) on iPlayer.]
3x06 Episode 6 [series finale]
A lot of people seemed to think this was clearly a season finale that someone thought they could get away with leaving as the last-ever episode. Personally, I thought it rounded quite a lot off, just not in a very cheery way -- essentially, all the bad guys won and our heroes lost. Perhaps writer-creator Peter Moffat was making a point about the justice system?