Apple describes Siri as...
[An] intelligent assistant that helps you get things done just by asking. Siri understands context allowing you to speak naturally when you ask it questions, for example, if you ask "Will I need an umbrella this weekend?" it understands you are looking for a weather forecast. Siri is also smart about using the personal information you allow it to access [...] (Source)
There are three operative words in that description, which I've emboldened, giving us more than enough reason to believe Siri's answer to the question, "What is the best smartphone ever?"
Flaunting a massive 4.3-inch AMOLED Nokia Clear Black display in front and a powerful 8 MegaPixel camera with Carl Zeiss optics at the back, Nokia Lumia 900 wrapped in monoblock polycarbonate shell could well be considered the Finnish company's true flagship release this year. On the software side, the phone runs its Windows Phone Mango 7.5 Operating System with its 1.4 GHz Scorpion CPU and handles multitasking with 512 MB of RAM.
So I guess that pretty much settles it. Unless you have objections, of course.