I skied Sunlight on Dec. 31 and Monarch on April 2. I will absolutely defer to anachronism on this. I judged that Monarch was sufficiently remote to any sizable population base. But I totally get that Sunlight is "in the shadow" of the Aspen areas, just like the 3 Utah places I listed vs. their famous neighbors.
I disagree on CB. Most of those 350K visitors are on about 600 of those acres. The North Face terrain is great, but rarely is all of it open, and I'll bet that surface lift has a big line when the snow back there is any good. More of Telluride's acreage is usable, so I suspect it's less crowded per acre than CB most of the time. The "big name" resort on the list that most tempted me to include is Sun Valley. It also does in the 350K range and has massive lift capacity, so no liftlines. It's a big mountain too, though with erratic snowfall there are times you're confined to the groomers as at CB.
Regarding Monarch, at first glance it looks too remote, but remember that it is the closest ski area to Pueblo and a toss-up for Colorado Springs skiers vs. Summit- a toss up that doesn't include I-70 and is thus much more of a consistent drive. Most of Monarch's skier visits get pulled from the ~1.5 million population that live on the Southern front range.
I love the place, and it is a place where snow lasts a lot longer than the visits per acre stat indicates, but I would be lying if I threw it up as a legitimate contender here.
And yeah, in April Monarch gets very deserted. It also tends to get hit hard with late Spring storms. My birthday is April 4. For about a decade from 2003 to 2013, I would stockpile Colorado Gems cards- card that give the holder 1 free day at Monarch in April. My birthday party would be taking my friends skiing. 9 out of 10 times, I got a birthday blizzard, and it was always deserted.
On the flipside- Christmas to New Years was the only time I EVER ran into any type of line at Sunlight that wasn't created by an extended chair stoppage.