I can't PICK... I watch a lot of youtube trainers. lol. And I like them all for different reasons.
I really like
Zak George for his enthusiasm, and I love the way that he talks and interacts with dogs and I love his overall message and vibe. His newer videos are definitely more descriptive than his older videos though. Older "how to's" IMO were never all that thorough and he used to only show his BC's who would get things much quicker than most dogs would. I think his newer videos are put together very well (and in a class setting with lots of different dogs) and great for beginners or newbies to dogs.
I like
Kikopup for her trick videos and how to's. She breaks things down into little steps and shows you exactly what to do and is very precise. If I want to teach a certain trick, I will check her videos first. But I find her very monotone, kind of boring, and a little bit TOO 'serious' at times.
I also like
tab289 - his videos are also very well put together and organized and he explains himself well. Sometimes the parts with him talking are SO up close to his face though and it kind of creeps me out, LOL.
Oh, also really like
pamelamarxsen ... sometimes the sound is a bit low on her videos for some reason, but overall she is a very good trainer IMO and I think she's maybe somewhere between kikopup and zakgeorge. Not quite as serious as kiko but not quite as 'goofy' as Zak George.