It depends on what you mean by LP. GameGrumps aren't letsplayers, jus two or more guys playing games and talking about it/something, and they are one of my favorites. PBG has a gameplay channel where he does LPs, but they are different and better than a normal LP, and his awesome.
But the god of LPing has to be Chuggaaconroy! He puts a lot of dedication to ALL of the games he plays, and does it for us, the viewers. He does an amazing job to entertain and informs us in a great way about the game (ex. Pokemon) and he shows games that are really awesome, even if it's a challenge to him (ex. Kid Icarus: Uprising). But the best is he wants us to be part of his experience (Animal Crossing New Leaf)

No offense, but seriously? I completely hate Gronkh, like pretty much every other german LPer. What do you like about him?

BasilZero

I don't really care for them or the title, But the only person I enjoy watching play games is DashieXP. Since DashieGames is just one of his secondary channels, he plays games for pure enjoyment and usually just plays random games. It also helps that I find his commentaries on the games hilarious.

http://www.youtube.com/dashiegames

I think he's even surprised that so many people watch him play games.

This. 

Chuggaconroy and AttackingTucans.

Shinobi-san said: BasilZero

^

Chuggaconroy is basically the only let's player I keep watching. Also ProtonJon when I have time to watch his streams...unless Luchajin is there, she is extremely annoying and not funny at all.

The only one I've actually sat down for to watch is Nerd cubed.
It's great comedy to see him take on some obscure pc simulator without first reading the manual. His ETS2 videos are a lot of fun too.
It's kind of Zero punctuation doing let's play videos.

I see he has a GT6 video up, that won't end well :)

I guess it's not a let's player in the sense that none of the games he plays can be finished in the traditional sense.

davidr64yt, or X, is quite good. If it weren't for him I wouldn't have wasted thousands of hours on Minecraft. Bless you X!

One F Jef is another favourite of mine. Probably not that well known since he seems to avoid mainstream games like the plague but it's very entertaining to see him take on ultra hardcore games (and fail miserably).