I have always found Rackspace excellent but they are expensive. They are often best if you are not going to be able to rely on IT Operations being available as an internal resource. I.e. when I was IT Manager for a small team and I was also IT Operations so having their backup was great.
Dedihosting in the UK have been good too. Slightly cheaper but not quite as high spec as Rackspace.
As a business case I was able to sell Rackspace as it meant we could hold off recruiting for a while and it certainly meant we had access to efficient, attentive and skilled staff 24/7 rather than me fiddling about not really understand all I should. For example when our site got caught by junk mail filters they put in place a few things straight away that cured it. That could have taken me 2 days.
I would guess some of the other providers are just as good however, I just don't have experience with many others. I have used donhost but they sound too "cheap" for you in that you are on your own to some extent (although 1/10th the price).