I'm going to try to give you a sampling what I think might be the best of the YouTube gun channels, although I'm still sort of new to it, so I don't want to give the impression that this is "the definitive list."
AK Operators Union - If you like the "Evil Black Rifles," and/or torture tests, these are your guys.
Forgotten Weapons - Mostly covers old and odd firearms, and how they work. Usually revolves mostly around military firearms, but sometimes sporting guns.
TheYankeeMarshall - A lot of Right-To-Bear-Arms videos, laced with midget prostitute jokes. Sometimes he does videos about revolvers and pistols too. (EDIT: Nevermind. TYM is a fool, and not even the good kind)
The Nutnfancy Project - I have mixed feelings on this one. He gets in some of the hottest new stuff - IMI Tavor, Kel-Tec KSG and PMR-30. The UTAS-15 shotgun review was pretty good. However, I get bored with the shooting videos. Watching someone shoot at steel target off in the distance that you can't even see is kind of pointless. TheYankeeMarshal cracked a joke about how he's not going to be doing 40 minute videos, in full battle-rattle, about a $12 knife, and I immediately knew exactly what he was talking about. He did 5 videos, ranging from about 40 minutes to over an hour on building first aid kits. Probably one of the most popular gun channels on YouTube though.
MrColionNoir - Kind of the opposite of Nutnfancy. He seems to accomplish more in two and a half minutes than Nutnfancy does in 45. A lot of Right-To-Bear-Arms videos, which has a lot to do with why he got picked up by NRA News. He's also done videos on the Saiga-12 shotgun, after-action-reports on training classes, and other good gunnie stuff. Calls himself an "urban gun enthusiast."
Regis Giles (a.k.a. Girls Just Wanna Have Guns) - The female MrColionNoir? (EDIT: 6/25/16 - Has been very little activity on this channel)
TFB TV - YouTube channel for The Firearm Blog, which you should be familiar with.
TheFirearmGuy - He makes videos about some odd and unusual guns like the Mossberg 930 and Taurus PT-22. He also does some Right-To-Bear-Arms videos.
Military Arms Channel - Sturmgewehre reviews a lot of the hot new Black Rifle stuff. He's a Kalashnikov rifle guy, but does a lot of AR-15 and pistol videos too.
Mrgunsngear - Prolific reviewer of black rifles and pistols. Another Kalashnikov rifle guy.
Hickock45 - He goes to his spot in the woods to shoot steel targets and bust 2 Liter soda bottles, cinder blocks, and clay pots. Guns range from new stuff, to surplus guns, with an occasional retired police revolver thrown in here and there.
John Lott - Author of More Guns, Less Crime. You may have seen him on TV.
RURdy4it (Are You Ready for It) - Get the news on gun legislation.
Sootch00 - Sometimes reviews guns and gear. Sometimes talks about guns laws, prepping, ammo shortages and magazine prices, and other things relevant to shooters.
The Gun Collective - A news show.
MattV2099 - I've posted on some of his videos on how to make bump-fire stocks. If you like watching videos of soda, Justin Beiber standees, saltines, and hams getting shot up, this is your guy. He's also done a lot of crazy Glock torture test videos.
CarniK Con - I would argue this is the funniest gun channel on YouTube. Um, try to imagine if Uncle Rico from Napoleon Dynamite did Magpul Dynamics type training videos. It's kind of like that. (EDIT: 6/25/16 - Quit YouTube?)
Dynamic Pie Concepts - Hasn't been active, but there are some funny videos on the channel if you're just bored.
fxhummel1 - Songs about other gun channels, gun laws, gun range dummies, fanboys, and tacticool fools. (EDIT: 6/25/16 - Now Jordan Winkler channel. He's now working for James Yeager's Tactical Response training company. Does more gun and shooting videos, and hardly any music anymore.)
My YouTube Favorites playlist
My RKBA playlist
If you think there are any glaring oversights, or if I screwed up any links, go ahead and post a comment.