This is a tough one and there are many options available that offer different things to different users. However much of your decision will come down to your personal requirements and what the intended usage and applications will be.

However here is a very quick summary to get you started.

iPhone OS:
Pros: Exceptional user interface and applications store.
Cons: Must deal with iTunes and Apple's control issues, plus only available on the one device.
Good for: Apple users and those who like things nice and easy.
Link: http://www.mobicity.com.au/iph...

Android OS:
Pros: Innovative and fast growing, across a wide range of manufacturers with different features.
Cons: Varying versions control across different handsets and basic Microsoft Exchange synchronisation.
Good for: Google account users.
Link: http://www.mobicity.com.au/pho...

Windows Mobile:
Pros: Great integration with a Windows machine, providing exceptional sync options with Exchange. Excellent for bespoke business applications.
Cons: Most UI's are unfriendly and hard to drive. Lacks a decent on-demand application store.
Good for: Large businesses and Microsoft Exchange users with shared calendars, etc.
Link: http://www.mobicity.com.au/pho...

Symbian OS:
Pros: Affordable and available in a range of mid-level handsets. New Ovi store picking up ground fast.
Cons: Predominantly available in Nokia handsets only and quite basic functionality.
Good for: Teenagers and the value-seekers.
Link: http://www.mobicity.com.au/pho...

BlackBerry:
Pros: BlackBerry are the mobile email pioneers and offer the best level of business-grade email.
Cons: Relatively poor deployment of touchscreen and other multimedia technologies.
Good for: Email junkies.
Link: http://www.mobicity.com.au/bla...

For more information on other OS and to find out which handsets offer your chosen OS visit www.mobicity.com.au