Within two hours of Seattle you have, from closest to furthest:
Snoqualmie Pass (The Summit) with Alpental as the gem. Alpental is small but has some pretty spectacular steep stuff and back country. Problem: low altitude and iffy weather. When you get it on a good day it's wonderful. It's within 45 minutes of a lot of the Seattle area, so you can get there in a flash. The other three areas at the Pass are ski rinks. I did most of my learning there in the 60's.
Next in line for most of the Seattle/Everett area is Stevens Pass. A nice area with good snow fairly often. There are some good steeps and a lot of terrain. My favorite lift is 7th Heaven (named WAY before Blackcomb's). A short double that takes you up a cliff to the top of Cowboy Mountain. Weekend crowds are insane during the ski scool season (Second weekend in January until late Feb.). Stevens has the largest area open for night skiing anywhere I've heard of. I have a mid week season pass that I got for $200 when I bought it before May 31. Works on weekends from 3/1 on. I have a cabin near there and it gets me up there fairly regularly, especially for spring skiing.
The last and most well known is only a little farther than Stevens and closer for those on the south end of town and Tacoma. That is Crystal Mountain. Amazingly I have spent little time at Crystal, mostly because of the cabin at Stevens. It was rated at #5 on this year's Skiing Mag's top 25 ski resorts. It must be because they have opened up a whole bunch of new territory (at least I think so). I don't get to Crystal at all anymore because it is farther from my house than Whistler and given a choice, I'll go north.
About 2.5 hours from the middle of Seattle is Mt. Baker, now my home area. A throwback that is not at all corporate. Don't go there if you don't like boarders. Skiers are in the minority. It has the world's largest average snowfall and locals often won't even get out of bed for less than 6" of new. Lots of cliffs and drops and steeps. Only 1500 vertical, but there is a lot there. If you're an out-of-bounds skier you'll love this place, but you'll need to have back country training, a buddy, and all of the gear. They lost 5 people in several different incidents in 98/99 and they became very strict on this. No lodging at the area. Long, winding road. Not a resort, but #17 on Skiing's list.
I have never skied White Pass. It's a long haul from the Seattle area. It serves Yakima and Southwest Washington more than the big urban areas. If you looked on a map and it looks like it's not much further than Crystal, look again. Those roads are closed in the winter.