what buddy now?

Ampharos?

probably pokemons for mewtwo raids. i don't think other legendaries require a nisch team of attackers so Yeah, mewtwo

I'll assume eevee and eggy's are also taken care of so:

Scyther/scizor would be good for Mew(too).
Rhyhorn/Rhydon(ground versions) for Raikou (maybe-depends on biome)
Then look to future evolutions Swinub, tangela, etc.

Then some lower tier options could be houndoom, phanpy, Omanyte, Jynx, legendaries, & whatever makes you feel good. At least that's my plan.

Maybe Porygon for Porygon-Z...or other future gens..

Doesn't really matter too much now if you have every mon you need (eg: 6 Tyranitars with both Dark/Dark).

Could even do Pidgey like a lot of high level folks for level spam when they eventually increase the level cap.

In all seriousness, pidgey? I just changed my buddy after reading this topic as my goal is to reach level 40.

Pick a fun project to max out. Something completely unconventional. Eventually, I really want to max out a Pinsir and a Tauros just because those are 2 of my original favorites.

I choose unown as buddy. Just because I think it is cute and I don't have many of unown candy. That's all.

Use those pokemon which you don't have evolved form in pokedex.