Beyond each of us saying the one we use is the best :-), syncing is the key feature as Ozavanti mentioned. I am a Mac user and hence have an iPhone 4. Adding the MobileMe service to it, it is a dream. I enter a meeting on my Mac and it appears over the air on my iPhone… No crashing, no problem. And … no Windows 🙂
Now if you use a Windows based laptop, then you’ll have to study which device synces the best. To my knowledge, iPhone works well also with a Windows based pc.