Sinnoh Stone Situation

Why not wait till something becomes actually useful? And maybe make a couple of the same ones and actually get some use out of them? How is really any supposed to answer this question with out knowing what other Pokémon you have? It seems like you are just doing it for the Dex entry so just evolve the one you like the most

Have you read GPs article on the subject? https://pokemongo.gamepress.gg/sinnoh-stone-tier-list

If your lineup doesn't have any glaring holes, just pick your favorite.

The answer is obviously Magmortar. Because team Valor has a fire-based theme and every member of the team should show his support. Her support, too, I guess. And if you happen to be not a member of team Valor? In this case you did screw up your game so hard, it really does not matter at all. :-P