pretty much any of them.
Just dont expect any support at all as no one understands macs.
What you want is one that is not bundling in a "broadband modem" as many of those dont work with mac. Just nip to Dabs or similar and buy a router.
Similarly a "wires only" service ie with no attached email is cheaper as a lot of the free software is not mac compatible. Get a .mac account £67 i think. It integrates seamlessly with your mac, and you can have a range of email addresses
with @mac.com on the end which is cool.
eg
[email protected]
www.forum.macformat.co.uk/ and
http://bbs.adslguide.org.uk/postlist.php?Cat=&Board=mac
offer good advice.
I use bulldog they are ok as long as you never have to call customer service. My parents use nildram again no big problems. I use a Router (netgear) to connect to my ADSL line and as far as the ISP is concerned that could be connected to a PC or a MAC it makes no difference.
I then run a wi-fi network in the house for my powerbook.
12" G4 powerbook, 12" G4 ibook. iSight. etc etc etc.