I might be in the minority here but I'm not a coffee expert or coffee snob - it mostly all tastes the same to me. I go to coffee shops purely for the atmosphere and a sit down - the drink is kind of just an aside. Out of all of the main coffee shops, in terms of atmosphere and friendliness of the other people there/servers I'd say:
Caffe Nero
Starbucks
Costa
Personally, Costa has become the the McDonald's of the coffee shops - there's one everywhere now and they're in areas that aren't so great so the clientele is noticeably different from Nero which I've never seen anywhere else except city/town centers in good areas (i.e. not on high streets next to pound shops, like Costa). Nero seems much more directed towards the professional person or the smart student. Costa has a daytime-tv-watcher vibe about it.