Rising Forces Online.
God, I love that game. I have never ever in my life experienced multiplayer like that again.
There is something so epic about 3 armies of hundreds of players working in cohesion under the unquestioned authority of 1 leader each.
When you see the line of warriors with their shields forming a wall, and the mages and rangers ranking up behind, as a tide of enemies advance on to your position... Amazing.
It is the ONLY game that has ever felt like truly epic multiplayer combat.