Wishbone might be a nice choice. I wouldn't exactly call it quiet, but I've always found it easy enough to hold a conversation there and I have a hearing deficit. Likewise they're usually busy but I've never felt rushed. It's easy to find street (meter) parking nearby.
Or, for something competely different, Manny's Deli. Not really quiet, but I've never had trouble hearing my tablemates there. The down-scale, cafeteria thing might not be what you want for a business lunch but, again, parking not a big problem and it's convenient to the loop.
And, in a nod to the bizarre, this note from Manny's web site: "Manny's Now Has Wireless Internet available throughout the entire dining area."
In both cases, real good food and real Chicago experiences.
Wishbone
1001 W. Washington Blvd.
Chicago
312-850-2663
Manny's Coffee Shop and Deli
1141 S. Jefferson Street (just north of Roosevelt Rd.)
Chicago
312-939-2855