I can't believe I'm saying this, but the Geth Juggernaut... with the Mantis I. Started new manifest on PC and got the Jugg as my first rare - very disappointing, but wasn't going to let him go to waste. This is a very aggressive way to play him and allows you to quickly become proficient at reload canceling and headshot execution.
Build: 0/6/6/6/6 (no hex shield). Spec turret for all top row (damage/flame), siege pulse for radius, DR, resistance damage. Fitness for all bottom row.
Protocol: Shoot turret just shy of spawn. Flamethrower will stagger > Headshot mook > reload cancel with Siege Pulse > fire explosion / more stagger > headshot mook > repeat siege pulse > headshot cycle until all siege pulses are gone. Be sure to redeploy turret before activating siege pulse to get your charges back.
I've never played the Jugg on my primary manifest, but I've always hated how many Jugg users would spam that ****ing hex shield right inline of my right-hand advantage from softcover. Using this build, you are much more of an asset, and only have to act as the standard aggro-sponge jugg during objectives or when an otherwise defensive position is being overwhelmed (e.g. camping PuGs). As an added side bonus, you *should* get the 20 headshot medal nearly every game.
Also recommend using the Javelin or Widow over the Mantis. But all I have is a Mantis I, QQ.