I don't have to assume. It's obvious the Legion is by far the most evil choice.
For the player, sure. All you see is the terror they inflict to their enemies on the NCR side. You are barely ever let into Legion territory in NV and when you do, it is really only their war camp. If an entire nation was judged solely by how their victims of war viewed them, then every country would appear evil.
Fact is, the Legion provided stability to the wasteland. In a brutal and semi-psychotic way, sure. But it is clearly able to continue providing that level of stability, while the NCR is showing clear signs of over-stretching. When the Raiders attack, or the Powder Gang breaks out, or when the Super Mutants go on a rampage... which side is going to better protect the people?
The answer is, of course, House. But barring that, the Legion is, long-term speaking, the more likely choice. The NCR can't continue at the pace it is going and when it falls, it will leave its people even more exposed than ever, because they will assume they have security or backup when they really need it and they just... won't.