Personally I think listing various youtube accounts is rather pointless in itself - wouldn't it be better to actually provide some information about the accounts, what types of videos do they make? why do you like them so much?
In no specific order, here are my top 5:
seanharry - organizer of the conventions I attend who posts videos of his events, some of them events I've been to - great for reminiscing about it
Half0utLoud - makes these crazy amazing Castle fanvideos, really great vidder
ImprovEverywhere - this insane improvisation group that does stunts around New York to secretly film people's reactions - my favorites include their 2 minute freeze at grand central, the reinacting of the opening scene of Ghost Busters at the NY Library and invisible dogs, lol
pheobecolefan - probably my favorite vidder of alltime, her work inspired me to want to do better on my own vids
ppplily - just a fabulous vidder, discovered her through the vidder mentioned above and she does great vids for several of my favorite fandoms