I'm sticking with my Brewmaster because it's still fun. That's always my primary concern, though I have mulled a swap to Bear or Warrior because of how strong they are.
I agree that Brewmaster is a lot of fun.There's just something a little unsatisfying about tanking as a brewmaster for me, though. I particularly dislike how they can't mitigate smaller incoming damage very well. It's easy to take a big hit and stagger it and purify it into nothing, but when you're pulling a lot of smaller mobs continuously and taking a bunch of small and medium hits, purifying doesn't really work out and you just take a lot of damage. I've talked to a Brewmaster doing Mythic+ 7-8 on beta about this and he said that the solution is to keg smash and kite things in circles. That's not really what I want out of a tank.
I'd say...
Druid > DK > Warrior, based on how fun I find them, and how enjoyable/flexible they are with the off-specs (Druid's are obviously gods).
In terms of strength for Mythic+, it's possibly the other way around right now. Warriors are ridiculous.
I think it's inevitable that ignore pain gets nerfed by a lot. So I'm not putting too much stock in their current power standings. I still like warrior because intercept and leap are so fun to use, though. Shockwave is one of the best abilities for 5s in the game too, in my opinion.
Druids are great, and I've said it before, the 'smart' choice for legion is always druid. Moonkin is great. Resto is possibly the best healing spec for all types of content, feral is really good in certain situations, and bear is a ton of fun and very strong. You can't go wrong with a druid.
I really like blood DK, I'm just afraid that the lack of mobility will wear me down over the course of an expansion, especially when I'm focusing on 5s where mobility plays a bigger part for a tank.
Honestly I'd be happy enough with any of the choices, I just need to buckle down and choose one. I don't want to play legion like I played WoD, where I had every class at level cap and never really invested into any of them.