I don't religiously watch LPers, I like to come back to them if they're doing a series I'm interested in, and I spend a lot of my time watching Let's Plays. The ones that stick out the most in my mind are some that have been mentioned by others here too.
At the moment, PewDiePie is my most favorite LPer. His commentary's awesome, and the reactions that he makes are priceless. Generally, he's just very entertaining--he never fails to make me laugh. :] Another of my favorite would have to be Chuggaaconroy. Some people may not find his jokes to be the funniest, but... yeah, he's definitely an amazing LPer. Along with these two, I also like 2JoshJepson, 3NintendoCapriSun, 6AttackingTucans, and Achievement Hunter.
Chuggaconroy. I used to watch Lewis and Simon on the Yogscast a lot (their Shadow of Israphel series specifically) but it's been months and I just be bothered to go and check out their stuff and try to catch up. But Chugga is enjoyable and makes awesome comments and it's just nice to relax and see his videos sometimes. And he's partially the reason why I actually got Animal Crossing: New Leaf o o p s. But how he managed to pull off his AC: NL let's play was amazing okay. I really really enjoyed it and was actually sad to see the first half end haha. Overally though Chugga is the one LPer I go back to from time to time to watch more of his stuff since I haven't done so yet.
> Pewds and Chugga definitely! Heck, Chuggaaconroy is the reason I bought AC:NL! I forgot how much I love Animal Crossing.
> Attacking Tucans and Josh Jepson are amazing in their Versus Series. I loved their Mario 64 and Ocarina of Time runs, though they are pretty crude but once you get through that they're hilarious!
> TheSyndicateProject for his Minecraft and CodZombies runs. I think not counting VEVOs he's the #1 most subscribed UK'tuber.
> Yoteslaya was pretty cool when he was around. Another CODZombies player. Yes, I may have had a mild obsession with CodZombies. But RIP, Yote
>MrRoflWaffles. A CODZ player. Plus his name is so much fun to say!
>MrDalekJD. CODZ again..
> Cry (or ChaotikMonky or Cryaotic). Damn, that voice. His LP of CorpseParty was downright amazing. Seriously, if you like lets plays then why haven't you checked out Cry?
>My Poketubers include: Haydunn, HardyT3kYoyo, NateWantsToBattle (All the Yes. Seriously) and TheJWittz.
Yes, I spend a lot of time on YouTube, OK? But seriously, out of all of these I recommend Cry the most.