Since everyone's covered a lot of big names, I'll concentrate on lesser known channels which deserve more love in my opinion;
-Ainulindale21 (One for the nerds who enjoy following religion/evolution debates on youtube. Great content and freaking hilarious)
-Aspreyroberts (Seriously some of the funniest sketches I've ever had the pleasure of coming across. Higher production value than most big name youtube channels too. If I had to pick a favourite channel, it would be this)
-TheHamWallet (More funny stuff, especially "Pokemon but with animals instead")
-Ragenineteen (animator that's worked with a lot of the big british names like Bing and Tomska, his own channels great as well)