My absoloute worst is anything like an LHR-CDG, LHR-BRU, LHR-AMS cause they're just too short and rushed and YUCK YUCK YUCK YUCK YUCK! But mainly the LHR-CDG cause the passengers are just sooooo rude, and please, please, please when I ask you if you want a drink, DO NOT REPLY IN FRENCH! I know these people speak English and we're rushed enough as it is to play their silly 'I refuse to speak your language' games! When they really need something, amazingly their English becomes perfect!
P.S. I'm probably one in a million, but I LOVE the ATH and DME and flights like that!! Give me anything that's long! I need time!!!