As a new server player, I can relate. I'd choose Yagura.

My opinion :

His mystery, being able to copy one of your ally ninjas is very niche. He must be very fun to use, you can try various teams and strategies.

My reason to pick Yagura is, he has unlimited potential, because his role is mostly a support. For example, imagine copying your friend's Pain 6P or Guy Gate of Death (although this doesn't happen often). The difference between Yagura NY and normal Yagura is, with the former, you can choose whom to copy, while the latter, the enemy chooses it. This mean, you own better control.

Besides, Mei is just a typical position 1 that you would use until you get better ninjas (just for collection or secondary p1 afterwards).

Although, the downside of Yagura is, he can only starts copying from round 2 and it costs 40 chakra. Furthermore, the clone may be squishy and require 3 round to be respawned again (no cd reduction at all). Himself can be squishy if you use him as p3-4. This make me doubt if we can utilize him in real battle. Maybe use him as p1 when your ally has better ninja?

Don't forget his mystery is not solely to copy ally ninja. His first mystery can inflict defense shatter, which can potentially increase your chase. This made me ask whether he will synergize with ninja not only with good passive or standard, but with good chasing skill aswell, such as Obito Rage? However, I don't know if this will work in real scenario. Let's say you can successfully keep two Obito in your lineup. Would the copied Obito does decent damage? because I think the stats of copied ninja descend from your Yagura.

Well at least it will certainly work in arena.