To be fair, his gear alone, so called non class abilities based on corruption, trinkets, breath of the dying essence, weapon procs and trinkets will kill players instantly in 5 sec no matter how good they are that is what pvp has become. Here is a breakdown: Pikaboo Burst
DH Havoc is situational, depends on the opponents class merely, some get simply countered, some will have an easier time. If you can stun and burst, like a sub rogue and got the gear, they are no match whatsoever, if you lack burst and hard cc you are doomed as demonic DHs will not die to high sustain or rot dmg. Leech will be reduced in endgame zones lvl120 when there are a lot of high hp mobs around, while leech will be insanely high scaled, when its an old zone with low level mobs like outland.
DHs can spec into Tank basicly removing their weakness to burst and with enough HP i wonder how strong twilight devastation tier 3 will probably be as its HP% based on the user, might 1 shot people with this and breath of the dying, while still have high selfheal and be one of the mobile tanks out there.
In that light i consider havoc quite tame and manageable, as long as you do not play a class like warrior, enhance or spriest.
Other than healers and tanks, my vote will be VoP feral and sub rogue(with duel).
Once they got the gear they will burst people from 100 to 0 and have quite the tools to manage multiple people, burst 1 and escape and swiftly come back for more.
But basicly you could just play any class by abusing scaling mechanics(low lvl mobs, high fire mage ignites of 90k per sec are possible) and gear, corruption and essences and use a glider to escape quickly.