I will second Chicago, although the time I ran it, I ran American Develipment, so the logistics were a little better - but the start and finish at Grant Park should be pretty convenient no matter what.
The flatness of the course, the crowd quality, and the depth of the field make it tough to top.
That being said, I've also heard great things about Grandma's. Not flat, but not hilly either - just rolling and just enough to engage different muscles at different times, keeping serious fatigue from setting in.
I also support NYC, but the course simply isn't fast. When I thought I was only going to run one - that was the one I chose to run since I lived along the course in Brooklyn as a very young kid. Spectators were even better there than Chicago. Logistics suck though.
As an aside - I ran 2:35 in bad weather in NYC and 2:30 in Chicago, and in both cases, I ended up running alone a bit over last 10k. Mostly because I was fading, but even 40,000-50,000 people races string out some even modestly towards the front.