A-basin usually challenges Loveland for the first resort to open. So either one of those resort would be a good bet.
But here a good article that you might find informative, this guy is pretty good. He knows numbers very well, everything is based in information from previous seasons.
EARLY VACATION PLANNING FOR OPTIMUM SNOW CONDITIONS
RULE #1: Don't commit money to a ski trip until the snow is on the ground. No one can guarantee blue sky powder days, but in foul weather or spring conditions you can make adjustments and still enjoy skiing. If the runs aren't open, you're S. O. L. This advice applies most emphatically to expert areas, where more coverage is required and snowmaking is limited or nonexistent.
Eastern skiers can apply this advice to New England, taking advantage of great early seasons such as December 1995, while avoiding the warm or rainy years. However, for skiers visiting destination resorts beyond driving distance, we can recommend the following areas for early season skiing.
RULE #2: If you must plan an early season trip (before mid-January), the following areas, in likely order of reliability, are your safest bets. Crowd considerations may also affect the peak week of Dec. 26 - Jan 1.
1. Mt. Baker, Wash.
Mt. Baker is the only lift serviced ski area in North America to average over 600 inches snowfall per season. They are often open as early as Halloween. Baker is entirely a locals' area. Vancouver is about 2 hours away.
2. Grand Targhee, Wyo.
Here it is the consistency of snowfall (and moderately pitched terrain) which makes full operation by early December a virtual certainty. In the past 20 years the lowest pre January 1 snowfall was 76 inches, and there was only one other year under 110 inches. Local skiers come from Idaho Falls. Most destination skiers stay in Jackson, an hour's drive away. The earlier your trip, the more likely you should consider staying on the Targhee (west) side of the Tetons.
3. Mt. Bachelor, Ore.
This is the most promising large mountain (3,100 vertical, over 3,000 acres) in the early season. As a predominantly intermediate area, about 90% of the mountain is well covered by a 4 foot base. In the last 24 years, the January 1 base depth has been less than 4 feet 3 times and the December 1 base depth has been less than 40 inches 6 times. The biggest risk here is weather closure of the Summit lift (about 30% of the time). With 7 high speed lifts and Portland at 3 1/2 hours the closest large city, lift lines are minimal, even during peak holidays.
4. Alta, Utah
Powder Mt., Utah
Alta ranks below Mt. Bachelor because its dry snow builds a base more slowly and its steeper terrain requires more coverage. As more intermediate areas with comparable elevation to Alta, Brighton and Powder Mt. will have similar coverage in the early season with about 80% of Alta's snowfall. Alta has low lift capacity and therefore excessive lift lines at Christmas. Brighton has 2 high speed lifts and Powder Mt. is rarely busy.
7. Wolf Creek, Colo.
Wolf Creek's snow base averages are comparable to the above 3 areas in Utah and it is 2,500 feet higher in elevation. However, variability of snowfall is higher in Southern Colorado than Utah, so drought months are about twice as likely. This is a very remote area. Crowds are rare, but lodging within an hour of the area is limited.
8. Steamboat, Colo.
Steamboat is one of three major Colorado destination resorts to average 340 to 360 inches of annual snowfall. Its snow measurements are from the middle of the area while Vail's and Winter Park's are from the top. Also, Steamboat's skier density is less than the other two areas because it is farther from Denver. A good choice for Christmas as lift capacity is high relative to the bed base.
9. Winter Park, Colo.
Winter Park's extensive data indicates a very consistent and reliable area. In its worst early season, 1976-77, it still received 50 inches of snow before January 1. A favorite of Denver day skiers, though lift capacity is high.
10. Loveland, Colo.
Similar snowfall to nearby Winter Park, but more wind exposed with variable exposures. Very high altitude, comparable to Wolf Creek. Bypassed by most skiers on the way to larger Summit County areas.
11. Fernie Snow Valley
Island Lake Snowcat, B. C.
Snowfall is high, but low elevation can mean occasional rain. As there is no large city nearby (Calgary, 3 1/2 hours), the areas are conservative about opening until they can ensure close to full operation (mid-December in below average years). Although lifts are not modern, remote location holds down the crowds.
12. Whistler/Blackcomb, B. C.
Both areas are reliable above about 4,000 feet, and high speed covered lifts bypass the rain-vulnerable lower mountain. Glaciers cover the top 1,000 feet, so less early season snow is required to open the expert runs compared to other areas. The chief drawback to Whistler and Blackcomb before February is that daylight hours are restricted. Enormous lift capacity, but world famous reputation means high cost and crowds at Christmas.
Somewhat more speculative, but worth considering, are the following areas.
13. Vail, Colo.
Beaver Creek, Colo.
Vail's snow statistics are very similar to those of Winter Park. However, the back bowls require more coverage to become skiable due to both steepness and southern exposure. They are open by Christmas about 75% of the time. Beaver Creek gets less snow than Vail but has much lower skier density. Busy, and extremely expensive, at Christmas, but worth considering in mid-December and very attractive in January.
14. Summit County, Colo.
Snowfall is not that high, but snowmaking (850 acres in the case of Keystone) is absolutely guaranteed with base elevations over 9,000 feet. Summit County is also within day commute distance of Winter Park, Loveland and Vail. Popular with both destination and Denver day skiers, but there are several high capacity areas from which to choose.
15. Snowbird, Utah
Snowbird ranks below some areas with lower snowfall in the early season because of its predominantly advanced terrain. A 6 foot base is necessary to ensure good overall coverage, and it's difficult to avoid lower mountain rocks with a base less than 4 feet. Solitude bears the same relationship to Brighton as Snowbird does to Alta. Snowbird can have moderate lines at peak times. Solitude's lines are much shorter.
17. Jackson Hole, Wyo.
Jackson receives about 80% of Targhee's snowfall, but its expert terrain requires much more coverage. If you're lucky, you might get December 1996's 200 inch snowfall. If not, Targhee is one hour away. Jackson's lift lines are more a function of its old lift system than excessive crowds.
18. Sierra Nevada resorts, Calif. and Nev.
Sierra snow tends to arrive in massive dumps, with sustained stretches of sunny and mild weather in between. The volatility of California snowfall means that the major Sierra areas will be close to full operation at Thanksgiving on a 6 foot or higher base in the best 25% of seasons, but will be marginal to poor well past Christmas in the worst 25% of seasons. The major areas have very high lift capacity, and there are several smaller areas which are less busy. However, careful planning is necessary to avoid traffic and parking problems during peak holidays.
RULE #3: Consider driving instead of flying from the Midwest or West Coast to the Rockies for one week or longer trips. This strategy allows you to utilize Rule #1 more effectively. If you must set your vacation dates in advance, you can still take conditions into account in choosing your destination. It's nearly always possible to scrounge lodging on short notice or from cancellations, while last minute airfares can be exorbitant. Driving with a group will reduce cost or allow you to spend the airfare money on more luxurious lodging, snowcat skiing, etc. Besides, you know your equipment will arrive at the ski area at the same time you do.