100% mareep

Ampharos isn't too meta relevant, so the "correct answer" is to not power it up.

However it is a very cool mon and has 100% IVs, so power it up if you want to. You're playing the game to have fun.

I would have evolved it without hesitation during the event to get the Dragon Pulse move and the held in case it became relevant the future. I also had a high IV mareep that I missed evolving because it has three 14s and I only evolved ones where identified >80% with at least one 15. The other turned out to be my highest IV even though it had no 15. Oops.

Hell yes.

A couple local players, including myself, like to use Ampharos in raids. It always gets props for originality and its good enough, not the best, but it will do.

If you already have the dex entry and have a high Ampharos already then I would just wait in hopes that Ampharos gets better moves. None of its currently available movesets are very ideal for much of anything.

Just keep it as your trophy...

Congrats on the 100% Mareep! I also got a 100% Mareep from the event! (from one of the two 10km Egg's I got during the event, and started hatching in the event. The other 10km egg was also a Mareep. It was like the Xzibit meme was controlling my RNG during the event.)

Don't power it up. Evolve it, sure - I don't think Ampharos will ever be meta-relevant in Pokémon Go, so even if you run out of candies evolving it, that's not likely going to be much of an issue - but don't power it up unless it will make you happy to do so, regardless of whether it's a useful Pokémon or not.

It's a nice happy trophy, and not much else. Maybe if Ampharos were to be able to learn Wild Charge in the future, it might become a useful Pokémon. If you had Evolved it during the event and it had Dragon Pulse, it might be moderately useful. (I wish I had hatched my 100% during the event for that reason) But now, it's just a trophy.

For mine, I evolved it right away. I still have more than 600 Mareep candies after the event (evolved 4 in total up to Ampharaos during and just after the event, still have 600 candies. Double candy + Pinap + Community Day = Why Did I use So Many Pinaps?), so it was only done because I had the candies. I do plan on using it as a Defender simply to give variety to the area, not to actually make it tough for people to take gyms. But if I didn't have the candies to evolve it, I wouldn't be making an effort to evolve it - Ampharos means nothing to me personally. If it means something to you, that's a good enough reason to do what you want with it.

Volt Switch is the best defending move in terms of raw damage and energy, and Focus Blast may net you a surprise KO against ground users like Golem and Rhydon.

However, now that Groudon is seen more often, and Rayquaza/Latios is now a popular generalist, this niche may be going away. Think of it as a defensive Jolteon.