Yes, despite the fact that recent reports have shown that Windows Phone has only a 1.7 percent market share in the U.S., which is lower than the 4.1 percent owned by its obsolete Windows Mobile predecessor, the Nokia Lumia 900 has gotten good reviews (by some).
And Siri isn't making a judgment call of its own. Much of what Siri does involves an implicit search, and Siri bases the answer to that question on the results of asking knowledge engine WolframAlpha the question (which means even those who do not own an iPhone 4S can simulate Siri results).
WolframAlpha is using customer reviews to generate the result to that question. Ask it what the second best smartphone is and you'll get a TracFone LG500G no-contract mobile phone.
Ask WolframAlpha what the best iPhone is and it will return the Sprint iPhone 4S with 64GB of storage. Ask it what the best Android phone is and it will return the MetroPCS Samsung Galaxy Attain 4G no-contract mobile phone.
Nonetheless, even with the explanation in place, it is an amusing result.