ContraryToPopularBelief Welcome! 
When battling, the best Pokemon to use depends not only on the raid boss itself, but on its movesets too.
So take for example Kyogre, the current legendary.
If there’s lots of you it’s not too bad, but if you’re cutting it close to the wire with low numbers, the Pokémon you choose can be crucial.
Your best counters for Kyogre are Raikou (with Thunder Shock/Volt Switch and Wild Charge) or Vensoaur (with Vine Whip and Frenzy Plant/Solar Beam) if it doesn’t have Blizzard.
Raikou is great as it’s good no matter what moveset Kyogre has. Zapdos and Jolteon should be secondary to Raikou as they’re weaker but still good choices.
If Kyogre has the move Blizzard, you want to use electric Pokémon and Groudon. If it does not have Blizzard, you want electric and grass types.
Do not use grass types if it has Blizzard! Pretty much a one hit knockout.
Of course you don’t know what its moveset will be before battle but if it suggests grass types you can be pretty sure it doesn’t have Blizzard.
If you’re unsure what Pokémon to use, google is your friend. Google the raid boss and it will take you to sites with excellent counter suggestions
Do this enough times and you’ll eventually learn what types are good against others and what movesets work well in which situations.