100iv Magikarp, evolve or not to evolve?

Evolve, Gyarados is very unlikely to get a community day.

Evolve and max it out. Gary isn't going to get a Community Day - it's just too hard to catch enough Magikarp to evolve more than 2 Gyarados in a 3 hour window for the vast majority of players, and even that would require a Double-Candy for Catches.

It's unlikely Magikarp and Gyarados will be granted a community day - the shiny has already been introduced to the game, and Gyarados already has access to both Waterfall and Hydro Pump making it a fantastic water-type attacker. So, go ahead and evolve it I'd say and enjoy using your max :)

If you don't have a Kyogre, you have a gap that you can fill with the 2nd best option which is also far away from any other.

On the other hand, in my raid group there is an ultra-hardcore player that has maxed out one of his hundo Magikarp (yes, Magikarp, not Gyarados). He has won over 1000 gym battles with it (!!!) in an ultimate exercise of showing off :)

Answered above, evolve it. The following points stack against Magikarp getting a CD:

- Shiny already available
- 400 candy evolution requirement
- Not 3 stage (disregard Pikachu and Eevee days, those were special)
- Already has access to the highest DPS water attacks in-game
- Lack of signature main series attack to be added later

I want to interject a note of caution here.

In Gen 6 (admittedly a very long way away), Gyarados gets a Mega form, that should get a dark typing. We don't know the mechanics of this, and it might require evolution with a specific item.

I have one perfect Magikarp (level 30), and I haven't evolved it for that reason. However, I do have several other Gyarados that are quite good, and several excellent Kyogres, and if I wanted another Gyarados, I could easily evolve several other candidates with the 2200 candy I have. So my situation is very different from the OPs, but simply dismissing the idea that a perfect won't have a different use in the future isn't entirely accurate.

I do agree that the OP should go ahead and use that perfect Magikarp, unless he or she has access to others that are nearly as good. Trading for a lucky one that might not be perfect but is very close is another option, especially if he or she has Pokemon from July or August of 2016, and that will reduce the dust cost to power it up substantially.