I wouldn't recommend starting all the way back with season one. That's a lot of videos. 
And watching all seven seasons isn't required to understand what's going on because each season is a completely different world with completely different goals/plots etc.
Season six is a great place to start in my opinion. It was very eventful and fun to watch. However, there are like 100+ episodes per hermit (and over a dozen hermits in Hermitcraft), so if you don't want to/don't have time to commit to that, just skip to season seven. It's also been a great season.
As for who to watch, my two go-to guys are Grian and Mumbo Jumbo. They're both clean, insightful, and very very fun to watch. I've also heard good things about GoodTimesWithScar, Iskall85, BdoubleO100, Stressmonster, and FalseSymmetry. You are not required to watch several hermits at once. In fact, for the sake of your time and your mental health, do not watch too many hermits at once (I only watch two and it's still a lot to take in ). 1-3 at a time is a good amount in my opinion.
I would also recommend this other channel called Hermitcraft Recap, which is a fan-made channel that does weekly summaries of what each and every hermit is up to, so that you don't have to watch all of them to find out. It's super helpful.
So there we go: my very wordy advice on where to start. 
