Do you want a Reshiram? For PvE? Yes, it’s a good counter, worth getting, because yes, use the same one for PvP! While not ranked as high as Zekrom, it still is respectable, and it’s a fire that has water protection from it’s dragon typing.
What should you use? Some of these guys are harder to find, Garchomp was a community day, if you missed it, ask a friend, it doesn’t need an elite tm or anything. You could also get a Dialga from someone who has raided, and do a raid, then use Zekron and Reshiram against themselves!
Go to the official counters page for a complete list including Shadows and Megas to see how your Pokemon perform.