Well, for starters, pick the race you prefer the look of. You'll be staring at your character's ugly ass for hours.
If you want the best racials for PvE, it's easy, go Troll.
For PvP it's a little more complicated and since it's long, I'm going to copy-paste a youtube comment I wrote just yesterday.
Warning: it's long.
I think both Trolls and Undead are really good, I prefer the flavour of Shadowguard, Hex and Berserking though.
Honestly, Wotf is powerful, but it's not a necessity, plenty of people play without it (like, 80% of total population) and it's only useful against Warriors (long CD on the fear), Priests (you play out of range) and Warlocks (and god are they a pain).
But does a Priest really takes advantage of Wotf, compared to say, a Rogue or a Mage? I don't think they have the necessary tools to make Wotf godlike. It's nice to have of course, you deny a cc, but I don't think it's as strong as some make it out to be.
So I prefer to go with some healing reduction and more face-melting.
Damage wise, Devouring Plague with 500 SP deals something like 1200 lifesteal, which is good but not really Mana efficient. With Improved Vampiric Embrace you heal a bit over 1500. That's over 24 seconds for almost 1000 Mana (or at the cost of a 3 min CD).
Now Shadowguard costs 250 and deals ~750 damage total per cast with 500 SP (with 26% scaling instead of 33%).
For 1000 Mana (a bit more than a single Devouring Plague), you can cast 4 of them, that's close to 3000 damage, of which you heal almost 900.
The numbers get a bit ridiculous with all the % you get from Vulnerability, Shadowform and Darkness but you get the idea. Devouring Plague heals way more but deals WAY less damage in the end.
Also, Alliance has more ways to get rid of your Plague (Dwarves and Paladins for example) than Shadowguard (No Shamy Purge). Shadowguard also protects your other spells from getting dispelled which makes it a nice defence.
Blackout procs on every charge of Shadowguard, which is nice since it procs on melee, range and spells (not sure how it interacts with DoTs though?).
Touch of Weakness has been said to scale way less than we can see on many private servers and it only procs on melee attacks anyway, which is a bit sad.
Cannibalism is great because it's basically a second bandage but it's rather specific, you need to have a dead humanoid body near you, but it allows you to get some health back without having to drop out of Shadowform which is nice.
A lot of people underestimate Berserking, and oh boy is it good. On Priest at least. The only time a SPriest drops out of Shadowform is when they need an emergency heal, which equals to a 30% haste on spells. Your Flash Heal takes 1.1 seconds to cast and the Greater Heal 2.1 seconds. If you kept your Inner Focus, that's a big crit that heals more and costs less than Devouring Plague. You could also use it for a quick Mind Blast.
Some people use Berserking for quick Mana Burn but honestly, at full life with 2 points in Improved Mana Burn, you're looking at a 2.25s cast with 10% haste instead of 2.5. At 20 %, it's 2 seconds and at 30%, it's 1.75s.
Keep in mind that Berserking doesn't change its value after activation. If you use it at full health it gives you 10% and then if you drop to midlife, it will stay at 10%.
Also, Mind Flay doesn't benefit from haste, which could have been really cool.