I voted Windows 6, as I am using an HTC Touch HD. Nice big screen.
One little point, though: Please remember that WinMo devices come in two flavours: PocketPC and SmartPhone. The difference? PocketPC-style phone are the ones with a big touch-screen and minimal buttons. SmartPhones look like "normal" phones with a small screen and numeric keypad.
The HTC Touch HD is a PocketPC-style phone. I tried running the WinMo version of ZimbraME on it, but it is compiled with SmartPhones in mind - having to call up the on-screen keyboard just to move around the menus/mail is a right royal PitA.
So please, a touch-screen oriented WinMo 6 version would be nice.
