Thank bud. I cant say im an expert on where to go but based on my experience I can let you know what i thought was best.
Niseko in general has the most consistent snowfall, so even though its FULL of Aussies, which does take away from the cultural experience a bit, its still great fun and your more than likely to get a good dump. There is a place right next to Niseko call Moiwa, when the weather is good, big snowfall and not a lot of sun and wind has hit the area for a while, that place will be deep and have half as many people that Niseko.
Nearish Niseko is Rusutsu which I loved, we didn't get as much snow there when we visited, but if we did, its much quieter theres a tons of good stashes to play in.
Another top place to check out is Furano - its quite a way from Niseko but you could check it out on you way to or back from Niseko area. That place is empty, you can duck ropes all day and get freshies all day, its amazing! Be avy equipped though, some places can become sketch.
Juat realised you actually asked about 'when' to go.... but maybe this will be useful to you or others anyway. But end of February im told is the best time to go, its got a deeper base so pillow lines are bigger and more fluid and the areas are quieter. Hope this helps.
Thanks to everyone else. Is there anymore video critique out there? Would love some.