Honestly? To me, it's Robin.
Without her, we would NEVER learn the history of the Void Century, and probably wouldn't even know it exists. Luffy does not care about that sort of thing, but Robin very much does. While Luffy is fighting people and taking care of the main plot, Robin's always in the background. Completing the sidequests that will eventually earn us that True Ending.
Robin complements Luffy narratively in ways that none of the other Strawhats do. While Luffy is loud and boisterous, Robin is quiet and reserved. While Luffy is direct, Robin can be sneaky. Robin is a good contrast with Luffy.
Robin's had one of the better character arcs in the story. Going from working for one of the biggest villains in the series, to joining the crew. And then finally accepting them as her family.
Even strength-wise, Robin's no slouch. She's probably in the top half of the Strawhats in terms of strength. She probably can't beat Luffy, Zoro, or Sanji in a direct fight. But, she could probably find a way to escape, at least.
If you removed most any one of the Strawhats from the story, it'd probably be lesser without them, but it wouldn't really change the story THAT much. You'd have to switch some fights around, but it wouldn't fundamentally alter the story being told.
But, I feel like if you removed Robin from the story you'd have to change a LOT to still make it work. We wouldn't have anyone who can read Poneglyphs, so the Strawhats would have to find some other way to get to Laugh Tale. Which means there wouldn't be a direct reason for the Strawhats to fight the Yonko (other than Luffy just deciding he wants to).
Robin might not get as many big flashy moments as Luffy or even Zoro. But, I feel like if anyone's the deuteragonist, it's her. If only because she's the Strawhat most connected to the Void Century mystery which is at the heart of the larger story being told.