What is the best mobile Operating system?

Duh....smartphone environment is getting more crowded with more and more new operating systems. From Microsoft Windows Mobile, Blackberry O/S, Symbian, iPhone's OS, Google's Android and now Palm's WebOS.

Which one is the best from the following criterias?

1. UI experiences (reponsiveness, easy to use, intuitiveness, beauty look)
2. Features (view rotation, touch responses, supports to various wireless tech)
   
3. Multi-tasking
4. Development environment and toolkits (including rich sets of libraries)
5. Portability
6. Openness (open-system, open source, proprietary)
7. Price
8. Hardware supports
9. Availability to developers to play (at least comes with a simulator)