Any of the players in say, the typical Forrester wave report is fine. However, don't necessarily trust the report's findings...just for the list of names.

One big thing, to me. Call each as you would as a customer. Apparently some are easier to get ahold of than others.

Pick one where you can get a competent human on the phone quickly. Because when you're getting DDoS'ed and threatened by your bosses at 1AM, it's going to be a big peace of mind.