The first two that come to mind are BajanCanadian and Jerome whatever his name is. I've seen them both in-game twice, and killed them both every time :3 (At least, thats how many times I remember seeing them)
Then there's sirshmoopies, who chased me in water for like 5 minutes on Holiday Resort (I'd spoken to him before, so in the next lobby he said he cried on his FIRST EVER facecam when he killed me. He didn't upload that video, though :c) and then I also killed his team of 5 who had full iron, stone-iron-diamond swords when I didn't even have full iron, and then it came down to a 1v1 in DeathMatch, where he had an enchanted sword and chestplate :C
Uhh
ThatOneTomahawk, probably doesn't remember me
@
Easy_Soup - awesome guy, he has an amazing basement (go check it out sometime)
NoBigDealCraft (Not the actual channel name) - 3 hawt girls who started posting on YouTube, and I'm the only guy that's done a video with them yet
The Arena Master - I fanboyed when I first saw him in-game, and he added me on Skype c: and I was in one of his machinimas