The number one thing anyone should consider when choosing to play in the level 49 bracket is queue times. Right now, on West the queue times are shorter for Horde than they are for Alliance. On East, Alliance has instaqueues at peak times, while Horde can wait a very long time for queue pops. Therefore, I strongly advise you to choose your realm/faction with great care if you want to maximize the amount of time you spend actually pvping vs waiting in queue.
For those that want to play alliance, the main servers with strong premade teams are Whitemane & Grobbulus (West) and Heartseeker (East). Whitemane sometimes has long queue times to log in, and it’s on West, so if you’re looking to play alliance with fast queues, go to Heartseeker, there are two guilds of lvl 49 players there that play together. We do gear runs for people that need it as well.
For long queue times and playing with premades on Horde East, check out Benediction server, they have a strong team, and so does Mankrik, though Mankrik’s crew is smaller.
For Horde West (fast queue times) check out Whitemane or Arcanite Reaper, Whitemane is super high pop so I don’t recommend it, and Arcanite Reaper’s team is led by Bello who is trying to recruit more to come to that very low pop server. Bello also runs a discord server that both Ally and Horde use to communicate with each other across servers. Reach out to me for an invite.
Most class/spec combos are viable at this level in PVP, though I advise against playing ret pallies as their DPS is just not high enough in classic. Again, the main thing you want to look at when choosing to play in the 49 bracket is queue times, and the other thing is who your potential teammates are going to be.
GL HF please note alliance needs a lot more lvl 49 players to balance out queue times on east. I am enjoying my instaqueues but I feel bad for the horde waiting for half an hour or more.