Mt. Spokane - No Alabi
49° North - don't have a clue
49*North signature run would have to be Silver Ridge. It's a blue cruiser that goes top to bottom 0ff the #1 chair. The best perhaps Cy's Glades, might be the best tree skiing I know.
I'd say Mt. Spokane's signature run is Cattrack, the run that bisects the Chair 3 terrain and leads back over to 1 & 2. I definitely think it's the most skied run, and it's kind of emblematic of the experience there... ski a great fun run partway down, then back on the cattrack again. Especially when the bottom of 3 has thin/poor conditions, so you end up skiing the cattrack even if you want to lap 3.
Silver Ridge is a great choice at 49North... it's currently marked green, but I agree it feels more like a blue cruiser, and the ski area's requested permission to build a bypass around its steepest section for novices. I could swear there was a ski website out there that listed Signature Runs for many different mountains (maybe as determined by Tony Crocker?) but I can't find it. My recollection is that Klondike was signature at 49° North... that was an excellent choice back when you could lap it, but with the way the high-speed quad changes the mountain, not so much anymore. My favorites when conditions are right are Double Jack and Gold Rush, for fast fun empty groomer bliss... don't tell anyone!
I'd probably say Centennial at Silver... it doesn't get as much traffic, but it's so long and knits the old Jackass Bowl together with the rest of the ski area.
At Schweitzer, Pend Oreille. Steep groomed bliss dropping into that gorgeous lake.
Old former home mountains...
Mt. Hood Skibowl: Upper Bowl
Snowbasin: Sweet Revenge
Mountain High: Calamity
Telluride: Plunge (or See Forever)
I agree with both of these, though I never hit Plunge just right.