Christopher Odd, hands down.

Discovered him around the time XCOM2 got released, because I was looking for someone to learn a trick or two from. Learned a lot from how he plays tactical games, but stayed for his calm and down to earth way (I absolutely can't stand that hyper, over-the-top crap that a ton of Let's Players do). Have also discovered the one or other gem through his videos.

Can only recommend!

Oh, and I also watch a ton of stuff on game design due to an interest into game development and recently did a blog post on my favourites there. To quickly summarise that: The Architect of Games, Design Doc, Game Maker's Toolkit, GDC, Noclip, Razbuten

Robbaz - He is a Swedish guy I have watched for a very long time now. He has the ability of turning what could be mundane games to watch, into wild and crazy stories. Highlight series he's done are Skyrim, Kerbal Space Program, and The Sims.

He posts a lot less than he used to on YouTube, but they're great everytime he does.

Pre-warning - His videos contain a lot of dark humour, so may not be for everyone.

I like Quill18 as a non-hypey gamer who plays a nice array of games. He's mostly into stategic sims, but ventures out to other game types every now and than. He has a solid understanding of the games he plays, yet he doesn't cheese it.

Dunkey, Let's Game It Out, NakeyJakey, Digital Foundry, and Gameranx are the ones I watch most regularly.

The Architect of Games and Razbuten are fun ones if you like video essays, and Keenengraz is great for finding lots of insane free games to play. ThatGuyGlen has some cool, short docu-style videos on the making of indie games that are really well done.

I'm sure I'm forgetting plenty, but those are some of my favorites, for sure.

Mandalore, Hbomberguy, AmbiguousAmphibian, Jwlar, The Dadliest Man, MattKC, Oxhorn, Joseph Anderson, Your Favourite Son, Summoning Salt, Noodle, Avalanche Reviews, Monty Zander, Boulder Punch, The Nth Review, King K

Maybe some others, but those are the ones that jumped out in my Sub list!