The correct answer to that question depends entirely on a set of services you need your phone to provide.
If you are looking for a phone with abilities to access corporate applications, exchange email, SAP etc., synchronize data and information, web browsing and MS office integration, then you should look at the Windows mobile phones. There are a lot of available applications for that OS. In my opinion, the best WM phones are produced by HTC; they have a wide range of models from the really stylish ones to the really tough ones.
If in addition to that, you would like to assure advanced communication and collaboration between your users, then the best thing to do is look for a Blackberry. For these purposes it could be the best option with lots of applications and extended communication possibilities.
Finally, if you just want to provide an intelligent enough mobile phone, with internet access, mail access and files synchronization, then you can look at Android phones, Iphone or Nokia Phones.
This question is still open. It is hard to single out one device which could be the best one for all users and IT as a whole. It all depends on your need and not all IT need their cell phone for the same apps 😉