Funny, Wizard and Glass is my favorite of the whole series, and is actually my favorite book of all time. I realize that I am not in the majority.
I even know a couple Beam heads that skip W&G when they re-read the series!

Wolves of the Calla is a great book IMO. I love the lore and side-stories! I also love the "spaghetti western" theme...You become immersed in Roland's ka-tet and I think this book really deepens the relationship the reader has with each member of the group (making the next two books more powerful). W&G was about a ka-tet handling a bad situation in a western town, just like WOTC - but this is a whole new ka-tet!