Jonah is epic. The routine abuse he gets over his size is marvellous. I also like Roger Furlong (the bald congressman) for his horrifically casual foul-mouthed eloquence.
If you enjoyed The Thick of It, then check out the American version, VEEP. I thought I'd hate it, but its genius. The writing is so sharp and the one-liners come so thick and fast, you're almost missing the next one through laughing at the previous. Armando Ianucci is one of the writers.