Illidan usually has the best players in the world on that realm from time to time. That one almost always shows as world top number 1 realm.
Faction does not matter a whole lot in retail. But you can still get a slight bit of a benefit from choosing the right one. Warriors from Above is an Ally made guild there on Illidan. I think I forgot to mention that one. Been around for a very long time.
Warriors from Above - Guild Summary
That is them OP could try to join I guess. I have an inactive toon in there. I keep lying to them and telling them that I am coming back to play but I never do.