We answer this one because we love the question! Fruition, ChoLon Bistro, Linger and Root Down are not to be missed. Had the best best best burger @ Highland Tap & Burger, love the Pizza at Marcos. Yelp it!
I am a foodie and love to get out and try all the new restaurants as they open. Fruition is definitely near the top of my list. Some hidden gems: Deluxe on South Broadway, Root Down & Squeaky Bean in the up & coming Lo-Hi neighborhood, Table 6 off 6th Ave in Cap Hill is an oldie but goodie. Cuba Cuba & Cafe Brazil are yummy South American establishments. Best meal I've ever had in Colorado was in Boulder, Frasca is amazing. Compares to the D.C. fare I dined on when living there. Rioja mentioned below is good as is Tag next door on Larimer. Steubens on 17th is a great little comfort food establishment. If you like Indian food, the best I've found is India's Pearl on S. Pearl next to Sushi Den.
Almost anywhere you stop on Larimer Square, you will find a good to great eatery! I love Rioja, Panzano's, Barolo Grill, The Cherry Tomato...just to name a few! A couple of good sources for recommendations would be 5280 Magazine and Westword, in their "Best of Denver" issues. I've included a link to Westword's list!
I like Trio and the Chophouse. Trio has an incredible wine selection and cool jazz atmosphere, often with live music. Chophouse is a great place to meet and be seen, and a fun, energetic crowd of people in their 20's, 30's, and 40's.