Depends on what subspecies you want to hunt...
For Central Canada barren ground, I hunted with Adventure Northwest at Little Marten Lake in NWT north of Yellowknife...Had a wonderful hunt..Most of the NWT outfitters ( and there aren't a lot of them) are honest and well organized and have high success rates...
In Alaska, I've had excellent results with Bay Air in Dillingham and Lake Clark Air in Port Alsworth hunting the Mulchatna herd..However, the Mulchatna herd has declined in the last few years...I understand the western arctic herd out of Kotzebue is a better bet nowadays..Never hunted there, but one air service that I have heard mentioned is Maverick Air..You might check up on them and other services out of Kotz..
I'm booked with Jack Hume Adventures next week in Quebec...
www.jackhumeadventures.com. Never hunted with them before, but several of my friends have and they all say JHA is first rate...One friend is going on his 9th hunt with JHA and has always done well..That kind of repeat business is a pretty good endorsement.. I'll let you know in a couple of weeks how I make out..