The Fourmile Canyon wildfires raging near Boulder, Colorado are the worst ever in Colorado history, as measured by the 169 homes destroyed in it and 25 more structures damaged. It started a few minutes before 10 a.m. on September 6, 2010 (based on the first 911 call reporting it) near 7100 Fourmile canyon drive, and three days later it is about 30% contained.
At last count, there were eight people and many pets missing, but no confirmed people killed or seriously injured in the fire and no strong reason to presume at this point that those missing are dead, so it was not the worst in Colorado history as measured by casualties or acres burned (at last report, 6,365 acres, which is about 10 square miles). An 8,000 acre area with 800 homes in it was evacuated, so while about 3,500 people have been displaced, it is not the largest fire in Colorado history by that measure either.
By comparison, the Hayman fire from June 8 to July 18, 2002, the previous Colorado record holder for homes destroyed "caused nearly $40 million in damages, burned 133 homes and forced the evacuation of 5,340." The Hayman fire was a partial cause of a death of a woman from asthma and the death of five responding firefighters in a traffic accident. It burned 138,114 acres.