RayWilliamJohnson - have to admit, was funny the first 2 times. But it gets old really, really fast. He makes the same fucking jokes and never puts in any more effort than he always has, which is not a great deal. What's more, I suspect he pioneered the whole bright colours and massive words bullshit for the preview frame. At best he is a vehicle for people to see some comical videos without going to the trouble of looking them up themselves.
PewDiePie - I can't believe people actually will be entertained by this. Some of the least mature and more importantly, least useful/intelligent commentary I've ever heard. Rape jokes are all well and good, but simply yelling "Rape" frequently does not comedy make.
The Annoying Orange has been mentioned by others, and although I don't see it recommended much or anything, I do remember watching a couple of videos. It, too, was funny the first 2 times and those times only.
iJustine - ...she's just stupid. Honestly. I'm of course getting this primarily from her Portal playthrough, which in turn I got from other sources, but although all her content is insubstantial, those videos in particular were just the most brainless crap. In fact I suspect she had to deliberately act stupid just to prolong the series, she was using a walkthrough ffs.
Also Smosh, can't be bothered explaining at this point.
Tobuscus I actually am impartial to. He puts some effort in, and although the thumbnails are the most visually offensive things I've ever seen, there is some genuine comedy and some genuine wit that shines through at times. There is of course a temptation to coast on name value alone, which is what I see many Lets Plays by already large Youtubers as being, but really, he does work at producing decent stuff. It's just a bit shallow for me these days (says someone who watches Game Grumps).
Also, have to defend the Amazing Atheist a bit too. A lot of his videos are(/were) good and provide an interesting point of view. I happen to agree with him on a fair few things. But he does deliberately garner controversy and hatred, and what's more, is expected to hate everything and take every stance to the nth degree, so to an extent I'd say his behaviour isn't his fault.
But thankfully none of this really matters because all you have to do to not watch the shit is not search for it.