YouTube is mainly the place for Let's Players, and if I'm honest I only ever watch a couple. Markiplier is probably my number one favourite YouTuber at the moment. I also dig watching Achievement Hunter/Rooster Teeth stuff (haven't started RWBY yet though, and still need to catch up on RVB), sometimes I watch Game Grumps (used to watch JonTron, but I lost interest after his couple of Twitter fiascoes), and stuff from Polaris. I used to really love Toby Turner for general funny videos and his daily vlogs but lost interest in him too. I've wanted to start watching Cryaotic and CaptainSparklez but I honestly have no idea where to start. @_@
Other than that, I also like watching Extra Creditz, Periodic Videos/Numberphile/whatever Brady's up to nowadays, Vi Hart, Veritasium, and CGP Grey (the last three, when I remember). I used to follow Vlogbrothers but it's been a while. I also generally use YouTube for things like origami and cosplay tutorials.