Fairly obviously (I at least would think) my favorite tree is the birch. I love them, in all their varieties! So I am not even gonna ask you that question. Instead, I am going to ask, What is your favorite stone?
As for me, I like all of them! But, if you nailed me down…um, but not really, please!…. I’d have to say my favorite type are the igneous rocks. I love the high drama of lava being spewed forth and forming aa or other of the volcanic stones. Basalt is so cool, especially when it cleaves into octagonal ‘pipes’, and granite is just so much fun to study; I spent hours looking at all the different parts of this one stone!
But hey! That’s just me! I would greatly enjoy hearing why you chose the type of rock you did! Just throw down a comment!
Oh, and if you would like to explore more about rocks, one of the best web pages that will get you to all sorts of info is Rock Picture Index: Master guide to rock pictures and descriptions, written by Andrew Alden for Geology.About.com.