Whistler, Big White, Banff, or Sun Peaks?

Whistler:  Huge, every kind of terrain, great skiing.  Crowded, expensive, a small city.

Banff: Beautiful town and setting, good skiing close at hand (however, I've only been there in the summer).  You have to drive to the various hills, they don't come down to town.

Big White:  Known as an intermediate's hill, also known as Big White Out because of the frequent fog.  Not near town so you either have to live at the resort or commute.

Sun Peaks: Extensive hill.  Mostly intermediate but it has some expert terrain.  Like Big White it's a way from town so you either live on the hill or commute.

If I had to choose I would pick Whistler for the terrain and I'd put up with the urbanity.  Of course, that's presupposing that I could afford it.

I'd probably choose to live in Banff and ski at Sunshine, but that's just me. At Whistler it's not like you're going to live walking distance from the lifts anyway unless you're rich. The commute might be about the same. 

of the 4 areas you listed Banff and Whistler are by far the best for skiing.  Banff is top for beauty and Whistler top for everything else. But a "season" doing what? Living off the cash and just skiing? Or trying to live in one place and get some type of job to supplement the trip? If the later secure the accommodation and employment prospects first most resort towns fill up pretty quickly. You might wind up in none of the above.

If it is just skiing also consider Golden as a base. Kicking Horse is the local hill, Lake Louise, Sunshine Revelstoke and Panorama are within day tripping distance  

Add in Nelson or Rossland, Fernie too.