Striker for PvP, ideally. Fist of Havoc is pretty much an "I Win!" button. See a stupid Bladedancer spamming his Arc Blade towards you? Fist of Havoc. Voidwalker getting ready to launch his Nova Bomb? Fist of Havoc. Sunbro just revived himself after you killed him once? Fist of Havoc. Defender dancing around inside his bubble? Run in there and Fist of Havoc. Some jackass with a shotgun or fusion rifle turning a corner and trying to blitz you? Fist of Havoc. The lightning grenade also does a crapton of damage if you can use it properly, that is, sticking it to walls and hitting enemies around corners or behind cover with it. A well-placed lightning grenade can wipe out entire groups and then some. Defender also has his uses as well. The bubble can be used to ward off attackers, and the Armor of Light can render you nearly indestructible to all but the most powerful attacks when you're inside it, (this includes Arc Blade, sadly), and the sticky Magnetic Grenade is easily one of the best grenades in the game, but personally, Striker seems to be the more potent option, imo.