Not great Groudon

The 84% it has the best attack

I'd power up the 84 % one because better attack IV

Either 84% because it is the highest attack IV and a difference of 5 stamina IVs is worth only 3-4 HP,
or the 87% because Groudon's base attack stat is already incredibly high so high atk IV doesn't matter as much other than for collection value.

Are either of them weather boosted? That would reduce your stardust investment. Generally speaking, the 10 atk IV will need four more powerups to reach the breakpoint of the 14 atk IV.

None of the above

What are their moves? How many TMs do you have?

I feel you. I've done 24 Groudon raids and have yet to find one with 15atk, let alone one with with 15atk and 90%+...

I'd boost the 14atk one, as stamina isn't really a biggy on Groudon. I have one at 14/14/12 that I am going to boost if I don't find a better one during the last couple of days.

You still have some time to try and get more. But remember that an 80% Groudon is still better than 95% of the other mon in your bag. It's a beast.