Whilst To'omua is currently the best 10, the combination of Cooper 10 and To'omua 12 is certainly the best option.
To'omua has all the attributes as an inside centre, and Cooper thrives with a ball player outside him.
No brainer for mine
Well, actually, i'd say it's the opposite, given his form in recent years.
Cooper is a playmaker who needs lots of people running direct lines at holes, and he will turn professional defences into a colander if you give him that.
There is no one better in Australia at seeing who the defence is focussing on and then giving it to someone they're not. If you select a backline full of playmakers, rather than runners, you nullify his strength because everyone is then trying to use their footwork and skills and sideways running to create things on their own.
Give Cooper a very direct, straight running 12 and he thrives. Give him a 2nd flyhalf and he struggles, because what he's waiting for is everyone to just charge at a gap and make the defence commit, at which point he puts it out in front of the man who is hitting the best line. To'omua is filling that role. His playmaking skills are a bonus, but the primary reason that pairing works is because To'omua runs fucking hard, and straight at the biggest gap in front of him.
If you put a Giteau at 12, it all falls apart because the Giteau also wants it early and wants to dance and run around people and not smash through a hole, so it just gets shuffled out to the 13, who then needs to be a big bastard to make any headway or have a chance of straightening the attack.
That's why Lealiifano struggles at 12, comparatively to his form at 10, and why To'omua seems to step up a gear when playing 12. Lealiifano is a playmaker, To'omua is a Horan style 12 who's grown up playing 10 and wants to stay there. He can play it, and fairly well because of his talent, but he'll always have more impact on the game at 12 where he can take hitups, run over people, smash them in defence, and still have the ball playing skills to cover for a flyhalf buried in a ruck.