Depending on where your going (obviously). My general order of airlines goes something like this
Virgin
United Airlines
BA
EasyJet
US Airways
Ryanair
Air France
Fly Monarch
Virgin is easily the top spot. Ive flown with them now more than any other airline, mainly UK/US and they are top notch. Ive had a few incidences, 1 where i was severly sick and they managed to move passangers around and gave me 4 seats all to myself to lay myself down on, their new entertainment system works like a dvd player where you can select from around 200 movies and stop/start whenever. Their staff are really nice and friendly. Both economy and premium economy have decent leg room.
United are equally good, lovely staff, so so entertainment, but crap loads of leg room (at least on the flight I was on). Ive only flow twice with them, once economy and once business.