It really depends on what you want. I've never had a bad time in Whistler whether it was midwinter, late spring or July and August.
However, if you want the best snow, you're probably going to want to shoot for the January/February timeframe. Although, you never really know when the big storms are gonna dump, so anytime in the typciall winter months are your best shot at the best snow.
You can get great Spring deals on lodging in April, and may still be able to get some great snow. so if you have a tight budget, look for deals in the spring.
Also, if you want to avoid large crowds, find a weed that doesn't have something big going on. If you want to party, find a week that does have something big going on
. Telus Festival is pretty sweet, but busy.