supergreatfriend is my favorite LP'er. He specializes in obscure horror games with interesting gameplay features (new and old), reviews of horrible XBLA games, and demos of various indie games that he recommends. My favorite LP of all time is his detailed playthrough of Illbleed (Dreamcast). He has a great voice, a dry sense of humor, and his knowledge of and passion for gaming really make him stand out above the rest.
http://www.youtube.com/user/supergreatfriend
On the other end of the spectrum (in regards to tone), I really get a kick out of
raocow. He mostly plays Super Mario World hacks and indie games. His videos are hilarious, if you appreciate fast-paced, absurd humor and non-sequiturs...
http://www.youtube.com/user/raocow