Avaya is in over 90% of the Fortune 500, and has a longer and broader pedigree in VoIP than anyone else (by far). http://www.avaya.com
Nortel also builds a good product, and has major penetration into large retail markets.
Shoretel builds a decent product, a bit lower $.
There are a myriad of softswitch platform providers out there as well.
The most important issue in any VoIP deployment is to make sure the underlying network is set-up properly for VoIP traffic. All network paths that carry VoIP packets need to be QOS-enabled (quality of service) to prioritize those voice packets over other traffic, or you’ll have problems with a VoIP deployment.