Most races of the year have a massive "what if" attached to it because there are so few races where anyone who is anyone is always there, in form and going for the win.
The giro, vuelta, dauphine, liege, amstel etc. etc are all great events but I'd say its only really TDF, Flanders and Roubaix where your get nearly all the biggest names in that speciality of the sport in top form and racing to win.
The Giro is like Ghent-Wevelgem to the TDF's Roubaix.
For that reason the Tour will always outrank the Giro for me. Thats not to say that e.g. the Giro in 2010 wasn't a great race that I enjoyed immensely (and the Strada Bianchistage was the best bit of sport, any sport, I've watched for years) but without Contador being there there's always that nagging doubt.
When it comes down to it, I want to see the best in the world racing each other when they're at their best and the TDF gets closer to that than the Giro ever will.