Absolutely pulp era Batman and especially (since the op specified just ONE)
Robin.
I know this isn't a popular opinion here, yet the truth is Robin to Nightwing was a step down, the most interesting thing about Grayson will
always be that he was Robin.
Grayson's Robin identity will always have the most cachet; particularly Grayson's take (going back to the Golden age where he's the young ,devilmaycare, daredevil, solo taking on Zucco (his parents killers) and the mob, somersaulting, slingshoting and kicking mobsters (even the Joker) off city skyscrapers to their death! So much that Batman has to intervene to save
them, from Robin.
That's his greatest most defining story arc. Who he is.
This will always be the definitive version, who Batman recognized in himself, had to real in, and take under his wing (and Oath), so he wouldn't become what took his parents.
DC has forever since been trying to retrieve that, with all these crap "new" derivative re-imaginings of the character, so they can have the young version back, and new readers can tell themselves look how edgy and "kick-ass" Robin is now, with mostly (creepy clones) trying to imitate the original. LOL!
With an inherent ongoing sliding-time-scale being the lifeblood of comics characters and why they've lasted so long, (despite some fans here pretending aging them is the way to go) One of the worst moves DC did was aging this character to Nightwing, when the only update he needed was a costume, adding green/black leggings cape and full boots.
Which Neal Addam's pretty much came up with for Grayson anyway, and every "new" incarnation has shamelessly with out credit been appropriating since. : )
Other than founding the Titans which he can still do. Most his extraneous supposed "character development" people cling to like it defined him, has been garbage.
Eliminate all the derivative clones, (if you want tell story beats where they stood in for Robin while he was unavailable, yet eventually took their own IDs that works great too) Truth is Grayson despite some delusion here, will never replace Bruce Wayne as Batman, and you or any writer will never come up with something more interesting than his role as the original Robin. He needs to take back and own his most important and significant role (that everyone from Todd, Timm, Carrie, Hit-Girl to Damian want to be), the one he defined.
Also his visual and thematic nod was both to the Bird and Robin Hood.