I guess it all depends on how big your budget is. Another factor is what region you want a server in. A server company might have great servers.. but if you're on the other side of the country or on the other side of the pond.. it might not be the best server for everyone.
Xfire me and I'll point you in the right direction.. The best server company I've worked with is Wolf Servers for the [SL]/~FL~ server. GWL works with Hypernia and another company in Texas for our servers. Good server companies will usually have multiple places to choose from concerning where you want your server/s located.