I don't have as many cookbooks as you (500? Wow!) but I do have probably about 20 or so.
For me, I always prefer ones where the recipes are quite simple in terms of the number of ingredients involved and the availability of those ingredients (nothing worse than opening a cookbook and seeing; "First take your Morroccan swamp grass and combine it with the yellow ends of your West Samoan green bellied snake fillets"!)

.
Not sure I have an absolute favourite, but I do like most of Rick Stein's books and Antonio Carlucci is pretty good too. I would have to say, though, that Delia Smith's 'Complete Illustrated Cookery Course' is the most used cookbook I have. It really is very good as a source of basic information (How do I make a cheese sauce? How long do I cook a leg of lamb for? etc) and also includes a lot of decent recipes.
When are you inviting us all over for a meal by the way?