Winter tires for sure for Whistler, but also for Big White. That drive can be nasty in winter. Hopefully your car will see winter/snow driving more than just this one trip so the tires will get reused.
There are several options for your 18" wheels but the top choices are Blizzak LM60 (best for snow/ice, less performance handling), Dunlop Wintersport 3D (good all around), Michelin Pilot Alpin 4 (best for wet/dry, good in snow). Whatever you choose, get on it right away. Winter tires sell out.
BTW, I'm jealous of your road trip. It sounds awesome!