4 kings I think. I didn't find anyone on NG+ particularly hard, but once I hit NG++ and beyond the 4kings became the toughest fight. Just so hard to DPS them down. This is likely because on my only NG++ character I level capped at 120, so I couldn't get crazy scaling damage on them. Might be different if I kept levelling. I think I made it to NG+5 before losing interest.
"Life is like a box of chocolates. Diabetics can't enjoy either."
either gwyn or ornstein and smough. four kings aren't a contender if you're poised up (unless we're talking about low level challenge runs), a piece of cake for an int/faith buff, decently easy for a dex or str build using power within. ornstein and smough are no challenge to an int build with multiple casts of homing crystal soulmass. it depends on your build basically. having the option to switch to the most ideal SL120 build for each fight i think gwyn is the only one i'd be at any risk of dying to. if we include the DLC i think manus and kalameet are both harder. manus for his single fast horizonal swipe attack and kalameet for the rising up and breathing fire straight down move.