First, I’d stay in Boulder rather than Denver. It’s just such a cool town. Beware that it’s apparently cursed (you’ll never want to leave….).
Winter Park is probably the closest and was pretty good when I went there in the late 90’s.
Crested Butte is awesome though and I had my best day on a mountain bike there (Trail 402 followed by 401) and I’d recommend anybody to go. Great little town too. I’d go back in a heartbeat given the chance.
Monarch Crest is on my list to do before I die and I kick myself I didn’t know about the last time I breezed through Salida.
A word of warning though, Winter Park and Crested Butte are located at 9,000 ft (and climb to 12,000) so it take’s a bit to get used to sucking the thinner air. Make sure your first day is a relatively easy one. I did about 6 miles on top of WP and blew big style. Fortunately the Yanks have handy mountain top restaurants to help you recouperate though 😉
The last time I went to CB I went via Leadville which has a cool (and flat) trail around Turquoise Lake which might be a good intro (thats at 10k ft though).
Enjoy whatever though, I’m jealous….
ETA: iirc CB is a 5 hour drive over the mountains from Denver