My favorite book of the 42? That's a toughie... I got REALLY REALLY into the Dresden Files series, so if we count all 12 (plus short story compilation) books as one, I'd have to go with that one. I miss Harry Dresden.
Apart from that.. well, Game of Thrones is really awesome. It gets a bit tedious with the sequels, though, it's worth it, methinks, but stilll.. Martin has GOT to stop describing every little detail for pages, especially if HE BURNS THE ENTIRE THING 2 PAGES LATER. Especially with food... the guy describes like every single meal, with detail, you can see why the fucker's fat. Awesome books, though.
Although you'll notice I also read all 11 books in the "entire" saga of Asimov's Foundation. Every human being should read these.
Damn, it's hard to decide.