Donphan 100%, is worth power it up?

Not unless you love Donphan. I powered up a 96%, then months later hatched a 100% Phanpy. Not going to do anything with it.

I have a 100% Donphan from a Phanpy I hatched like half a year ago. It's still level 20.

I took my 100% all the way up to the max breakdown point for Raikou back on the day. Other than that, it doesn't currently have any specific use. Groudon makes a better investment

Gotta love Donphan's design! Im keeping my 100 in hopes that they give him a ground quick attack one day.

well... he's better than my 98% Sandslash.

you don't have very strong ground units right now for someone at L39. If you have 100's of rare candy, your two Groudon would be far better ground units. if you are short on rare candy then I guess it may make sense to power up the Donphan.

Not right now, it's not that good of a ground attacker because it doesn't have a ground quick move.

If in the future there's a new ground quick move it gets that Rhydon doesn't, it could be better than it, but will most likely still be worse than Groudon unless this new move is on the same power level as the original 10 damage Water Gun.

I hatched a perfect phanpy also and maxed mine out, not because of its offensive capabilities(it’s a neat addition to gyms though with a good move set and defensive stats) but because I love Donphan and I can afford the stardust. I support powering up favorite or cool Pokémon regardless of cp or attacking powers.
Same reason I have a kingdra with double dragon maxed out.
So, if you can spare the stardust and like donphan, power that 100 up! 3022 is nice to look at.

If you love Donphan and have a lot of dust, then do it.

But otherwise, there's no point. Donphan is, at best, an above-average defender. I have one, and it performs competently on defense, but is severely outclassed on offense.

Invest in it if you want, but know that all you're getting is a passable defender.

Pretty usefull against dark types with C/FP

Donphan has some merit, but there are better options. Particularly with Groudon’s beast stats.

I have a 100% that I've powered up to close to max, I love it as a gym sweeper, but there are better options.

It's fun, looks great, but not one to power up if you're only focusing on the top tiers, as there's better options.