Hardest to defeat

Playing as both the Predator and as a member of the Fireteam, what kind of opponent do you find is the hardest to beat and why?
As a Fireteam member i know it can vary, especially when you end up with random people who might not be playing on the same level as you do. Premade teams will always be better than random people because of obvious reasons(communication, tactics, everyone’s role etc).

From my own experience, when i play as Fireteam, the toughest Predator i’ve encountered so far are the Scouts with ranged weapon specialization gear and perks who actually knows how to play. They constantly move and leap all around you/your team, causing confusion, scattering the group, building up the panic, and that’s where mistakes start being made by the Fireteam.
When i started playing this game i thought it was horrifying to meet a Berserker who simply ran into the Fireteam group and whooped everyone’s ass with combistick, today the Berserkers who attempts this are a joke, whoever the Predator is attacking draws his knife and starts parrying while the rest of the team downs the Predator.

When playing as a Predator i hate to meet Recon sniper groups who spreads out a bit and camps high grounds while one player completes the objective. It’s hard to enter the area and hard to heal up after second wind. They deal alot of damage, are fast runners and stay muddied up longer.
I’ve started playing as a Scout more and more because i understand how effective it can be if played right, though i need to practice more on not getting hit in my head by a branch when leaping, or make those leaps straight up in the air because there is an obstacle in the way where you stand. :P