I was just in downtown Chicago last month, and celebrated my birthday dinner at Frontera Grill. WARNING: They do NOT take reservations. We arrived at 6pm on a Saturday and waited for 90 minutes (they’d told us the wait would be 2 hours). Yes, the food was good, but I’m not sure it was THAT good.
I ditto the recommendation for Berghoff’s.
I’ll also recommend Joey Buona’s, for deep-dish pizza and other Italian food.
(By the way, if you’re looking for a fun/delicious place for breakfast, check out Toast in the the Lincoln Park neighborhood.)