What is the best phone for connecting to Microsoft Exchange Server 2003?

It needs to have wifi connectivity and a full keyboard (physical or touchscreen based) and be able to access an Exchange 2003 account natively, in particular the user’s Exchange calendar, and preferably other Exchange calendars (for example a companywide one in Public Folders). And I’d like it to not be insanely expensive so any somewhat older phones would definitely be an option.