Say what you will about Johns but he’s done a lot of great work plugging new energy (and marketability) into some classic DC properties.
I’d say Green Lantern 1st as that book ruled the world in the 2000s.
2. JSA - best run ever and made the Golden Age team important in the DCU again. Plus it helped to revamp Black Adam who has become an incredibly big part of the DCU and has a solo flick on the way.
3. Aquaman - the character still had a bit of the negative stigma before that run. Johns had Aquaman out selling Spider-Man and it was a huge influence on the movie.
For me personally, I’d put Legion of Superheroes pretty high up on list just because his Superman and the Legion of Superheroes storyline is one of my favorite comics. Perfectly captured the essence of the LoS and was also a damn good Superman story.