See this discussion for brunch recommendations: www.chow.com/topics/364403
For dinner, gosh there are so many great places, but here are some of my top picks: Piccolo Sogno for Italian, Sable for contemporary American small plates, Deleece on Southport for contemporary American, Mexique or Mundial Cocina Mestiza for provincial Mexican.
Even though they're both Italian, they're rather different in spirit. Piccolo Sogno is a more of an upscale bistro - still a casual place, but it's where people go when they want to have a nice dinner out, sort of a "casual finer dining" type of place. Quartino is much more casual, and is more oriented towards drinking as well as eating. The menu at Piccolo Sogno is quite contemporary in the sense of using modern techniques and ingredients, but traditional in the sense of offering appetizers, entrees, pastas, etc so you can have the typical 2-4 course dinner there (although they also offer individual pizzas and other lighter fare). The menu at Quartino is more of a "small plates" type place, with individual size pizzas and other Italian small plates. (You can view both menus on their websites.) If I wanted to have a nice sit-down dinner that would impress the most demanding foodie, I'd go to Piccolo Sogno. If I wanted to hang out with friends for drinks and light appetizers, I'd go to Quartino. Piccolo Sogno has the best modern Italian food in the city, in a lively upscale bistro setting. Quartino has a fun, lively, ultra-casual vibe for anything from just drinks to a meal. Hope that makes sense.
From a practical standpoint, Piccolo Sogno fills up its reservations well in advance (make your reservations as soon as you can); Quartino doesn't. Piccolo Sogno is in the River West area just north of Greek Town, right next to an el stop on the CTA Blue Line; Quartino is in River North near an el stop on the CTA Red Line. Both are open for lunch and dinner; Quartino stays open quite late.