it depends on the specifications you have for the ACD environment. I know Alcatel has a terrific ACD environment with different choices, starting from a OmniTouch Standard Edition via OmniTouch Premium Edition towards OmniGenesys for professional environments. Nortel has its own environment, just like Avaya has its own and Cisco.
As far as I know, only (omni)Genesys can connect to other environments, so if you want an IP PBX independant solution, you could choose for Genesys, however this is a very "heavy" solution. I think you have to focus on your requirements, and choose the one that best fits the situation.
Another point of attention is in what environment you want to deploy the solution, in other words, are you thinking of a purely voice environment or are you thinking of a (future) multimedia environment with chat, cobrowsing, mail integration etc. That will also influence your choice.
I hope this gives you a little direction, if you have any questions, please ask.
We are in the process of updating out legacy Toshiba PBX to an integrated Toshiba IP/PBX. I am stunned at the flexibility, Windows integration, legacy equipment reuse and ACD features. I am sticking with Toshiba because it has been reliable as bedrock for the last 15 years. The price is dirt cheap compared to the more "popular" solutions. Hope this helps.
If you are looking for a call centre or contact centre solution then Avaya is the best i this kind of environment..