iOS4 - iPhone 4
Apple have renamed the operating system, it's now called iOS4 instead of iPhone OS 4.
What's great about iOS4 is of course multitasking... finally. By tapping the home button twice you can choose which app you want to use and which ones you want to close. Nice and slick.
The OS as a whole is quick and unbelievably intuitive.
>> See the full review here <<