Each of the Macs listed have their positives:
iMac: Best value for money Mac available.
MBP: iMac performance with the portability.
MBA: Performance and portability. Makes an affordable MacBook if you can't opt for the Pro.
Mac Pro: Serious performance and it looks the best IMO.
Mac Mini: Affordable, great replacement computer, best computer to buy if you are switching to Mac but cannot afford to buy the iMac.
However, taking day to day usability into consideration, I would say the MacBook Pro is my favourite current gen Mac as it can be plugged into a monitor when you want to use a desktop or can be taken around, without the loss of performance either way.