In what order you should buy Vermintide 2 story DLCs:
Back to Ubersreik (top tier) -> Shadows over Bogenhafen (also good) -> Winds of Magic (arguably, sh*t tier)
With that being said, Back to Ubersreik is the best Vermintide 2 story DLC to this day, on par with Karak Azgaraz and probably The Curse of Drachenfels, which are free to play. Besides the Back to Ubersreik campaign itself, you get nice bonuses, like some of the best weapons for each character, and L4D2 Survival mode (even though it's only one map).
The only downside of Back to Ubersreik campaign is Olesya's constant interruptions during missions. I'd love playing the maps without being reminded that I must find runes (which I have already found) every single minute, thanks.