OP asked for Horde not ally !!
tbh stormscale EU has the most elite gamers atm but i consider it as a big trashcan hard to find a stable group 1 night u play nice 3's second day ur teammates are out of the realm just like hookers
in my experience right now i would like to assume illidan may have the best horde players but its not a 100% guarantee, the best could be tichondrius, frostmourne and now , kelthuzad for alliance.
illidan just has a massive horde population due to it being one of the first servers, that pvp video ganking funeral as well as the now defunct(?) blood legion guild but u will have a really hard time getting in there every time a new expansion comes up.
Alliance side of outland is better than the horde side.
If you mean "best" by the highest rated people then Stormscale, be prepared for the whole population to have massive egos and think theyre godmode though. Although thats the issue with most PvP realms i found stormscale to be bad for it. Also I would recommend outland because the queues are rediculous.
With MoP, I think we should look into possible PvP realms that have a closer ratio for world pvp.
They are not doing another WG or TB, but more like EP where we captured towers or other places in outland.
I think a good realm for this type would be Mal'ganis