I used to use Demon years ago when all their local access numbers ended "666", this was in the days before using 0845 dial up, every town had it's own demon telephone number.
Recently I've used Pipex - they were good
Screaming.net / Worldcom / Tiscali - crap customer service but this was a few years back.
I'm using Sky at the moment, service can be dodgy at times but their supplied Netgear router can need booting every so often - I'm also right on the limit for ADSL coverage which gives me a slow speed as well.