which Mewtwo should i power up?

4123 in that order.

If dust and candies truly aren't an issue I'd pick 1->2->4->3. Otherwise take Clurachan's advice.

Power them all up evenly, since they are useful right now for Mewtwo raids. Try to get them all up to level 30 quickly, so you can use them.

Delete all of them.

Jk for real though the second one

If you dont have already six SB Mewtwos powered up, then you should power up 1, 2 and 4. Power up 3 if you really need a fourth SB Mewtwo.

1,2,4 for a start. All three of them..
No, not to the max but just until it need 5000 stardust first, then use it.
You can power it up accordingly depend on your raid situation for example step by step to 6000sd , 7000, etc until, it maxed out, or if its already serve you well in raid, then keep it as it is.
Easier then using the raid simulator, and also easier to explain to fellow trainers. Most trainer in real life doesnt event know the basic of Weakness- Resistance- Basestats let alone how to use the raid simulation apps.