When I say most threatening, I am including many factors in addition to skill, perception being foremost of them. These factors include things like:
Respect (do you know he [there are no girls in MWLL, people who say otherwise are lying] is a good shot and a consistent even tempered player)
Hate (is this asshole always at the wrong place at the wrong time in the wrong asset in a way that gets you pwned?)
Activity (does this person play often enough that you even know they exist)
Asset Choice (do they vary what they pilot and generally pick respectable mechs and such or are they a jerk that always goes Vulture B on Solaris Arena; either way you have to keep your eye on them)
Rank (are they at/near the top of the scoreboard a lot, and often with a jealousy-inducing Kill-Death ratio?)
if you can think of any other factors to include feel free to say so.
My nominees in no specific order and from what I can think of at this moment:
AAA
Twinkletoes
Andreas
NotSoCoolJ

AIMB
apologies to my friends who I didnt mention here. remember this is as much about perception as it is skill.
edit- added AIMB