For me, 6Music. Its the reason why I bought a DAB radio for my kitchen.
Id would be so lost without it. Huey Morgan (my little slice of NY radio) and Jarvis Cocker on Sundays. Steve Lamacq & Marc Riley Mon - Fri nights helping me cook dinner. Craig Charles Soul & Funk show on saturday nights to keep me dancing...
Its the only thing close that comes to the station I used to listen to back home while growing up (its now off the air thanks to ClearChannel!) with its mix of new and old and random eclectic tracks.. Rarely theres a bad song on (at least imo)
Yeah, 6 Music is good. I listen to it sometimes online or off the satellite. I tend to get my music listening from other sources online though.
I love Radio 4 (and if you have a digital source/listen to the radio online Radio 7) because it is a bit like having a tiny listen to a by-gone era. I was an NPR listener back home (and for those missing NPR, you can definitely get most stations online, although I find the time zone thing and the accents a bit jarring now), but Radio 4 is like the real deal. Dramas, comedy, readings, panel shows. I love it. We went to a taping of a show last year, and I can say that it was one of the top 10 coolest things I've done so far in the UK. For those interested, you can get free tickets to radio and telly tapings, although I got the impression that there are some hardcore radio geeks who snatch up a lot of the tickets when they become available. A lot of them seemed to know each other.
Another resource for newcomers: All the catch up things online (4OD, BBC iPlayer, Channel 5 & ITV catch up). Some game consoles also have them as a part of their operating system now, so it's fairly easy to watch them on your regular telly even if it is not hooked to your computer.