Pat, isn't GGK from Montreal? So something I've been wondering since my teens, does he pronounce his name in French or English? I never know how to say it.
Well, to add my 2 cents: GGK was my favorite author for a long time. I discovered him at a time when I was reading poorer quality rip-offs of Tolkein, such as Brooks. Therefore, I have a soft spot for The Fionavar Tapestry . It's very flawed, a lot of it doesn't work very well (especially the Arthurian elements), but enough does work. AS high fantasy goes, it's perfectly respectable. Like later works by GGK, the characters are incredibly charismatic.
My favorite was Tigana, though I haven't re-read it in a few years. I suspect I might not be as enamored, particularly since my politics vis-a-vis nationalistic struggles has changed quite a bit since I was younger. From what I remember, it was the strongest work of his though.
Arbonne, Lions and Sarantium maintain a uniform level of quality, and you really can't go wrong with any of them.
I didn't much care for Last Light of the Sun, I found it slight in terms of plot, theme and character development, at least in comparison to his other works.
I also didn't think Ysabel was special, as young adult urban fantasy goes. His prose felt awkward when transported to modern day, and the character development was weak.