Sorry I'm late to the party. Today was my first day on skis in the past 1 1/2 months, we are welcoming our tiny human into the world- born January 12.
As for Monarch, of any place within day ski range of the Front Range, it has by far the least powder competition. Terrain is good enough but mountain is short.
I've skied Monarch for 30 some years, most of it as my home mountain. Last had a pass there in 2010, but still get a few days most seasons. One eye opening thing the past few seasons is they have low-key gladed out a TON of that mountain over the past few years. Pretty much the entire mountain has some wonderful trees- back in the 1990's there were a lot of snags and tight areas. basically pick an aspect and go nuts these days. Some standouts are Rock-O and High Anxiety Trees, and GBT- Gunbarrel trees skiers left of Gun Barrel.
The face skiers left of Panorama, with, Rock-O, High Anxiety, Jr's Zipper, etc offers the steepest terrain that you can lap off one lift. As a result, most of the locals beeline here first thing on a powder day. Still plenty to go around, but it will ski better in the morning. Even the local influence doesn't seem to go to the trees here.
Outback, Shagnasty, and Curecanti are good small powder bowls, but only with fresh snow. They mank up horrendously when the snow stops and the sun comes out. Shagnasty and Outback seem to be getting skied a lot more these days too..
Mirkwood is almost always worth the hike for windloading and snow quality. All of it is approachable. Orcs is the easiest line, Mirkwood proper has a great cornice entry and is one of the steeper lines (maybe 40*), Staircase is the most technical with some (avoidable) cliffs and couloir sections. It is a pretty short descent before you are on the return track, but if it is snowing, it is good.
As for Christmas Tree, Jerez is right, it is popular. Honestly, I've never really felt it, it is actually kind of a PITA traverse because you have to go uphill with skis pointed straight forward, hard to even duck walk. It skis out quick too, and the terrain is the same as lift served no traverse stuff. So, cheat. Instead of traversing above the lift shack, get off Garfield and hang a hard right into the trees immediately off the lift.. This is Sleepy Hollow Trees, gets skied a lot less, and offers a lot of the same as Christmas tree with no hike. Cross Sleepy Hollow and drop into the trees below for more goodness.
Powder day afternoons, I usually ski the Kanonen-Examiner Face. These two runs expand out into a lot of terrain with countless openings with fresh snow dropping down into Tango. It is really easy to get lift served freshies here all afternoon- powder bumps up top, untracked lines below.
I usually finish out the day with Gunbarrel trees to ski down to the parking lot- Especially now with our motorhome, as motorhome parking is right underneath Gunbarrel (and yes, Monarch is generally 100% ok with camping in their lot- just park under GunBarrel and follow instructions if you get any). On a powder day you will usually have to click out and do a short hike, but it is more of a 3-5 minute one to get to the top of Gunbarrel.
You won't like Monarch if you are unsatisfied unless you can ski fall line for 2000+ feet. You get 800 at a go. There is lots of 30-35* terrain but almost nothing steeper. Be ok with that and you will have a better powder day than anyone hitting up a Summit County resort.