Which should i should i evolve?

I wanna evolve my shiny magikarp but iv suck…
Luckily, I found a better iv magikarp (iv 91, atk 15,def 13, hp 13)but i am still wondering which one to evolve?!

You’ll probably get more mileage out of the higher IV one, unless perhaps you’ve caught it at a very low level. If it’s a similar or higher level to the shiny, go with that.

What might change the decision though is the availability of a like-for-like lucky trade. Getting the shiny lucky is guaranteed to improve it, with a minimum of 12 in each IV category. If you live in an active community, it’s probably not going to be hard to find someone to do this with.

There are better water types than Gyarados and easier to obtain, namely Swampert with Hydro Cannon. If you play with a larger community and have enough people for raids, IVs don’t matter much. You won’t see a difference between your two Magikarp.