I think it is simply impossible to pick one best song out of all the great ones. It also largely depends on my mood. And not only that, when I don't hear some good songs for a while they get in the spotlights when I play them for a while. But I'll write down some good candidates:
March of the Black Queen, Fairy-Feller's Master Stroke, Nevermore, Ogre Battle, White Queen, Don't Try So Hard, Innuendo, Play the Game, No-One But You, Princes of the Universe... Great King Rat... and I can make a list of ages. But just 1 best song? No, sorry, impossible.
Thinking just about the more upbeat tracks, I COULDN'T POSSIBLY survive without Living On My Own or Under Pressure. They send me into some sort of trance. Of the slow tracks, I love Love Of My Life.
Liar...so powerfull and dramatic...for me it's a real master piece. Of course we have many others like Bohemian, black queen and so on but Liar kind of grow on me trought the years.
Without a shadow of a doubt, The Prophet's Song. While I don't listen to it as much as I used to (deliberately so, as not to wear it out), no other Queen song gives me as much pleasure. It's not a perfect song- I usually skip over the middle part- but the first part is magical, and the conclusion is pretty darn good as well. :D
My favorite changes from time to time... right now it's All Dead, All Dead, at other times it's '39, or something else. Brian May's always been my favorite of the group -- his songs usually top my list. Freddie's are fun to listen to, but for some reason, I just don't enjoy them as much as I do Brian May's songs. :\