I suggest Earthlink. I had them for several years without any problems. They tend to leave you alone once you're hooked up, and they're national with lots of dial-up numbers. When I had dial-up, I never had busy signals (this is in Seattle) or problems staying connected. I'd be using them now for my DSL except that Verizon has locked them out of the area.
Tuor
Ditto on Earthlink. Started with Mindspring dial-up then upgraded to DSL. Verizon screwed Covad by not letting them share my second line. Covad had to put in a third line for DSL so I could keep my 2nd phone number. Earthlink bought out Mindspring but it is still pretty good service. Their system rarely goes down.