What are your favorite applications on Android or/and iPhone? And why do you prefer iPhone or Android?

I know that on iPhone there are much choices of applications, ok...
But if we just consider quality, and not quantity, which system is the best?
Comparison on useful applications and also videogames.
Also it seems that Android has much more free applications than iPhone?

And do you think that the Sony Ericsson Xperia X10 is a good cellphone, or should I buy another one?

Thanks