I'm trying to rewatch it all right now prior to receiving the 6th season on DVD for a 'surprise' Christmas present (NOTE TO MY MUM: It's not a surprise if you ask me what I'd like).
So far, I'm entirely undecided what series I like best. I think I can safely put 4 or 5 at the bottom. Some good episodes and character development, but they got too heavy on the storylines and neglected the one liners in favour of Gervais-still 'cringe events'.
I have a feeling Series 6 will be up there when I rewatch it as, on first viewing, it was immense. So I reckon it goes something like:
2 > 1 > 6 > 3 > 5 > 4
Series 1 - Mark Makes a Friend ('That was 'the bad thing'')
Series 2 - University Challenge ('There's no quim likes to party...')
Series 3 - Sectioning (or Shrooming) ('If YOU try to section me Mark you will have crossed a line and I will section you, so help me...')
Series 4 - Gym ('They don't know us at all...')
Series 5 - Jeremy's Manager ('Yes, I DO believe in crystal skulls!')
Series 6 - Bachelor Party ('This is all his roosters coming home to roost. It's a roosterthon')