' best race as a defensive holy priest '
It really depends on your play style ,
- Troll haste could be considered defensive since if you chain it with lifebloom you can land some pretty fast heal for the next 30 second
- Bloodelf arcane torrent is more of a offensive spell , you could argue that you can use it against casters to silence them mid cast and then take less dmg. But there is already very few casters in BGs and most of them are mage that won't get in your melee range most likely.
- Undead fear break is nice to keep your teamate alive when you get CCed , because eating a full 8 second fear sucks. Though it's not going to help you survive much if you are the one getting focused.
- Goblin rocket jump is nice to get away from melee , which honestly is what will hurt the most ( warrior and rogue ) also depending where you use it you can get pretty far away from your target if you are going downhill.
- Tauren aoe stun is mainly defensive, its pretty nice to get 1 flash heal off without being worried about getting kickced or even to run away from your target if you aren't slowed. It can also be used offensively to interupt a cast , the only downside compared to arcane torrent is that its not instant.
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I suppose since you said defensive holy priest is that you want to stay alive longer. I suggest going tauren as the stun and chatise don't share DR since one of them is a incap and the other 1 a stun. This combined with fear gives you alot CC against rogue even if they trinket your fear.
my second best option would be goblin, chatise then goblin jump away would work like a charm.
NOTE- that most of the racial have more use than what I mentionned , though those are the most common