Working on Stormblood now, about halfway through - good stuff. Debating what else I want to try to level besides WHM while I have the Road to 70 buff - so far just working on BLM.
Is there a tank class that has a good balance of aggro tools and mitigation that’s fun to play?
Every tank has more or less the same aggro tools and mitigation tools are fairly similar too, they normalized a bunch of stuff. I'd say the most "annoying" is still WAR due to Overpower being a cone instead of a circle, you can use the spinning axe move out of combo to gather mobs instead but it feels bad since well, it's off combo. As for mitigation, the "best" imo is DRK for casual content, but only after 70, when you get The Blackest Night, which is a great skill, but before that they're like super poop (relatively, they still can level fine) because they're more or less balanced around it. Finally PLD while great eventually gets most of his cool tools(proper melee rotation, heal, magic rotation) in HW or SB levels.
So the "best" choice would be GNB, who starts at 60 so you already have a well rounded character.
That said if you don't mind the low lvls being a bit boring, PLD is great too, and then if you prefer the aesthetics, WAR and DRK are definitely good, they just take longer(DRK) or need to play a bit better(WAR).
All tanks are nearly perfectly balanced currently and they have no glaring flaws I'd say, at least for casual content. For endgame, there's some edges but not necessarily that important either(if you play DRK you kinda want a WHM in your raid for the Living Dead/Benediction combo for example).