Mountains that i would not make an effort to go to unless then have a big dump are, Nakiska, Kimberly, and Panorama. In fairness i have only been to nakiska of those 3 however i have never hear good things about kimberly and pano.
Been to Kimberley and it's decent, but there are lift issues (lack of high speed and very narrow section at the bottom of the one high speed) that makes it not really worth going to for me.
Nakiska can have great days, and it can have shitty days. Obviously with less snowfall than most of the rest it's hardpack way more often than the rest. On the plus side it's not on many snowboarders radars, so with even a small dump of snow you can find great lines and some fun glade runs with almost no lift lines.
To the OP, you might be getting a little adventurous to want to do all that in 2-3 weeks. The drive from Fernie to Lake Louise or Banff is about 4.5 hours alone. If it were me, if you must see whistler, I'd fly into Vancouver and rent a car, do the whistler thing, then drive through the lower part of BC to Fernie (stopping at some of the resorts on the way as Snowolf suggested)... Plan to spend a few days in Fernie if the powder is falling, because it may be some of the best off-piste snowboarding you'll ever do. If it's raining you'll want to bypass Fernie and head North to Kicking Horse or Revelstoke... There's a place near Canal Flats, BC called the Lussier Hot Springs. I would strongly suggest a drive into the site for a free hot springs dip in the back woods of BC. The road in is amazing in the summer, in the winter it could get treacherous but the scenery will be fantastic.
After KH or Revelstoke (or both) I'd make my way to Banff and spend a few days there, boarding at Sunshine or Lake Louise. Of the two, I MUCH prefer Lake Louise. Sunshine has a lot of flat areas that require unclipping unless you're a pro on the flats. My GF wasn't impressed. I found a lot of decent runs but also a lot of issues with the place. Lake Louise offers something for everybody, at every level. Check some of my threads for pictures of the place.
For me I'm constantly flip flopping between Lake Louise and Fernie as my favourite resort. Fernie has the off-piste tree boarding hands down, but can get rain and is avalanche prone so the bowls may be closed for days on end. I've been lucky to get great weather at Fernie, and had my biggest and best powder days there. It's the resort where I've stopped in the middle of a run, looked around, and said to myself "this is what it's all about" more than any other run. But then I've had some amazing days at Lake Louise that were almost as good, and the lift situation is arguably a little better at the lake. Neither of them have 100% high speed lift access to the highest peaks. Some of the bowls in Fernie require 2-3 lifts to get to the top, and then the same to get back out. At lake Louise you can ride the gondola to the top, come down to the top of the world 6 pack and then over to the platter, ride the chutes in the back, come up the first quad and do it all over.
For fast laps Nakiska actually has my record. I've done some 35 km in 1.5 hours at Nakiska. Most I've done in a full day was 90 or so km and that was at Lake Louise. If I wanted to beat a record it would be at nakiska with it's 100% high speed lifts. I'd just get bored after 4-5 hours there. Hard to get bored at the other places!