First and foremost, Simon and Lewis:
http://www.youtube.com/user/BlueXephos
TONS of great videos, custom maps, mods, upcoming releases... this is the CORE video blog for most things Minecraft (at least the most entertaining, in my opinion), I think just about everyone here knows about those two.
Next up is Duncan, who is the Tekkit master:
http://www.youtube.com/user/Yogscastlalna
Sjin of course is the limey half of Sips & Sjin, lots of great videos that show you how to wrangle MCEdit and plan really cool builds, also has some fun Tekkit videos and of course hosts Minecraft with Sips a quite surreal series involving Minecraft and the great Beaver Mafia conspiracy:
http://www.youtube.com/user/YogscastSjin
You can't have Sjin without Mr. Underscore himself, Sips:
http://www.youtube.com/user/YogscastSips
A user some of you are already familliar with due to his music videos and perhaps some of his work on custom maps (Deep Space Turtle Chase FTW), Captain Sparkly-pants, err CaptainSparklez:
http://www.youtube.com/user/CaptainSparklez
Collectively there is a lot of content shared between all these guys but some really quite fun video series to watch, particularly if you are a Minecraft fan but also if you are a gamer in general who perhaps can't find all the time to check out every new game or mod (or map) out there.