Sylvanas is probably your best bet at the moment, decently populated with a lot of high end competition on the BG overall. Can't guarntee you the best que times coz it seems to be a widespread issue over most battlegroups.
Sylvanas hosts guilds such as Bruce lee which is a really well known guild and i think like one of the top rated on AJ. Arena Peak is also there which is really vastly growing. Overall there is a lot pvpers and the server is well populated but the game is kinda dieing so its not at its peak.
I played at darksorrow quite a while ago and still know a few friends currently playing there who are like 3-4 on the ladder atm. From what I can remember its not really as big as sylvanas but still is a great ally based realm.
I gave as much info for you as possible And I hope I was a little bit helpful!
it's a really hard game because when you lose you know its all down to yourself and you can blame it on your teammates or anything.
losing streaks are really hard to deal with because of this and you want to start cheesing, etc. Get someone to practice with and find some builds that work for you and get the timings down on them really well and don't just rely on being able to 200/200 a-move and win.
I think a lot of timing attacks are good if you get them down well as a lot of the people in bronze-gold don't have the ability to create as much stuff in such a short space of time. So if you can get a great unit composition fast or make a lot of units faster than him you will win a lot of games like that, by just being faster and having better macro.
TvT I don't know since it's gay and there are many different possibilities, just make sure you scan at like the 5;30 mark and see what hes doing so you know if you should get detection or prepare for mass bio
Shamans should have no problem outhealing casters unless the shaman is stunned (like deep freeze, in which they get completely shit on). Warriors/rogues completely shit on shamans however.
A shamans healing output is quite immense though when he is freecasting so he shouldn't have that much trouble keeping you up vs a rogue