I used to look after Bulgaria product for Neilson holidays. Been to all the Bulgarian resorts.
depends what you want really. Bansko is definately the pick of the bunch. as the resort is only 7 or 8 years old, all the lifts are modern and fast. ski area not huge but enough to keep you interested for a week as long as you're not some sort of mile muncher.
PLace is still a building site and will be for a while yet.
accom is pretty good, though the service levels in Bulgaria are generally lower than elsewhere (they are not intentionally surly or rude, but they don't really do smiling!)
Borovets is on the whole a bit older, massive hotels (1000+ rooms!). used to be a problem with hookers in the lobbys but not now. Ski area a bit more technical. and snow not as good as Bansko.
I woulndn't bother with pamporovo.
p.s. the snow can be excellent in Bulgaria, but it's a bit more of a gamble. I'd wait for a late deal and see what comesup.
pps – pre buy all your lift tickets/kit/lessons before you go. they will be at least double the price in resort. Despite repeated advice, customers turned up every week and got shafted, and there was nothing we could do to help.
enjoy!