It's got to be the Airbus A330 MRTT's. Used by quite a few of of our NATO Allies, is proven (unlike a KC-46), and is not a Boeing product. Air Transat might have some maintenance capabilities for it too as an A330 operator. Since it can do 3 roles (tanker, transport, VIP transport) the Government can say "See, it's not just for junkets."
Even in VIP Transport configuration, the UK MRTT's can still refuel planes. If Trudeau is smart he'll give up on the flashy liveries Harper added and stick with CAF grey to reinforce this fact.
Plus, there are a lot of used civilian examples on the cheap to pick from right now and sent through the MRTT conversion process. ("Look, used jets - whattadeal!")
Probably the right plane for Canada, but not having the fast jet contract sorted out 1st is terrible optically.