I think we should be aiming water laden comets and asteroids at Mars to terraform it.
Specifically I think we should be trying to increase the mass of Mars so that it can better retain an atmosphere. It’ll need a bigger moon too, to help even out temperature fluctuations and add some internal heating from gravitational pull.
Some of the mirror and laser ideas that are being presented for asteroid deflection could probably work. We could send a swarm of mirrors out to deflect the astroid Apophis which could hit us in 2029 if we don’t. And then let these go on out to the asteroid belt and start deflecting asteroids towards mars.
It isn't necessary to make every world a duplicate of Earth. Variety is the spice, they say.
The good news is that you don't need to increase the mass of Mars. It's a nice thought, but who wants to wait around for that?
If you supplemented Mars' atmosphere up to about Earth pressure, it would take about a million years for it to leak away again. That's the good news.
The bad news is that the best gas to use for the purpose is carbon dioxide. So you still wouldn't be able to go shirtsleeve without a breathing supply and mask.
Planet Venus offers a plentiful supply of carbon dioxide, free for the taking, and most folks think that removing it would improve that planet too. Take all you want. If it leaks away, go back for more.
For me, it's not so important that humans be able to breathe the atmosphere of Mars without assistance; but I'd like to get it so that plants would feel at home there.
Imagine the size of trees grown in Martian soil and gravity!
And don't worry about a bigger Mars moon. We'll orbit it with a Rotavator. That should be exciting to watch!