#3 - The Yukon River itself. We only paddled a bit under 1/10th of its full 3200km length, but every bit of it was absolutely breathtaking.
Many tributary rivers and creeks flow into it, and it gets very wide (and silty!) as they do, and that width really messes with your perception.
But it also makes it a great river to learn how to canoe, lots of room to make mistakes and few major hazards.