Starmie

If you have extra stardust and want to branch out and play with atypical mons (i.e., not Snorlax, Lapras, Dragonite, et al.), then by all means go ahead and power up your Starmie. After all, the point of the game is to have fun! That said, from a functional point of view your 100% Starmie won't ever be as good as an average Vaporeon (unless Niantic does some major rebalancing).

Lol. I, too, had a 100% Staryu I evolved yesterday for the same moveset.

Like the 7 100% IV mons I've evolved before it, they've all turned out to have sh*tty enough movesets I can't/won't ever use them, but perfect stats so I can't get rid of them.

It's basically just Niantic removing one of my Pokemon slots every time I hit the 'evolve' button on a 100% mon

My 93% has that move too...totally too slow and pretty useless other than prestiging perhaps, and even then, you are better off with a Vap, sadly...which can only get water gun.

I got a Starmie with 98% and water gun-HP and I power it up. To be honest, don't think I would do it again.

Tackle is very slow and very bad move. Do not power up any Starmie with tackle!

Definitly keep that 100% treasure.
I think you will not use it later on. With water gun it would have been nice, because it has the best dps of water type pokes.
In spite of being to high in cp imo for power up gyms, it would be the be the best solution to keep it on its level.