Extra Credits has to be my overall 100% favorite. They have a wide variety of gaming-related videos that are almost like miniature lectures on various aspects of gaming today, attempting to understand things like retro gaming, piracy, free-to-play games and educational games. Very interesting stuff. Now they do a bunch of other shows but the one I really like is Extra History, where they just talk about history; it's not even gaming-related but they do a good job of presenting information.
My favorite let's-play is probably Game Grumps and all the sub-shows on their channel, but Jontron is the best (besides Extra Credits which is kinda in it's own category). Some of JonTron's videos still make me laugh until I cry. PeanutButterGamer has a similar format to Jontron but isn't quite as funny IMO.
Yahtzee's personal youtube does neat let's-plays. Recently they did a few videos looking at some games that Yahtzee himself made when he was younger and that was hilarious. Gabe can get
really annoying sometimes but overall he's okay.
Retsupurae is pretty good, not for the actual retsupurae portion (where they riff on other LPs) but for their riffs on ancient point-n-clicks, FMV games and miscellaneous Newgrounds garbage.
I kinda like Markiplier. He just gets so excited for every game that he plays, but sometimes I feel like he tries too hard.
My least favorite is probably Pewdiepie & similar. It's the Youtube equivalent of junk food.