There is only one answer to this question... Geth Prime (even after the nerf) as I have stated many times before
This answer should be pinned or put in the FAQ section.
The geth prime is in a world of its own. It takes the crown for the cheapest enemy, most annoying and any other you can think of. Homing pulses that curve around corners (check), staggering combat drone that sees through cloaks (check), double turrets that go through walls (check).
Kiting them along with a army of other enemies is annoying (not undoable) since they are always last.
I can't believe that they actually used to stagger on every pulse on top of all that. :'O
Phantoms are a close second though; the on host DR isn't much of an issue for me anymore because I know the phantom's idiosyncrasies it is just the insane damage that the atlas palm blaster does coupled with the rate of fire.