Most of the national dialups IFAIK use UUNET dial up numbers. That means that you will get basically the same connection speed and the same access numbers for most of them. If I were in the market for a dial-up account, I would look at Earthlink and other similar sized ISP's.
I have heard that the larger companies get more bandwidth for their customers at the leased numbers, but I am not sure of this.
From my experience in ISP tech support.
UUNET has very good uptime, and in the areas that I dealt with, very few busy signals. To check if an ISP has UUNET numbers go to this
link and compare it to their dialup numbers.
GNAPS (Global NAPS) does not have very good uptime and has lots of busy signals. This was only in the New England area, but we had outages lasting several days for whole states.
Level3 seemed to be a fairly reliable provider, but we did not have that many subscribers using them.
Very few providers have 800 numbers anymore without significant per minute charges due to the cost. In fact, I do not know of any who offer it.
Good luck finding a dial-up provider, but go with DSL or cable if you can. You will never go back to dialup after that.