Naturally I have to say naga.
However, if I weren't tied to saying Naga, then I may be inclined to point my finger to dwarves or undead. Both have extremely useful pvp skills, and their other traits are also useful. Undead can be of any race, meaning that in theory they could benefit from the traits of any race, while retaining the bonuses of being undead. Dwarves are excellent craftsmen and soldiers, plus their building style is generally fairly fortified, making any attacks against Dwarven holds semi-suicidal (Expect for Dakkar, although i would want to see some battles as fire people that are weak to water sound easy to beat. Just dump a ton of water down their tunnels, and watch them die or drown. But thats just me disagreeing with lore.) which would make them dangerous opponents.
But thats more of a war focus on them. Other races do excel at other areas, Humanity and the Maiar both having excellent naval qualities (because the Regalian Navy is awesome.)