New World Franky is one of the most underrated characters in the
series.
When Doflamingo sent assassins to Punk Hazard to fetch Caesar, Franky solo'd them with ease.
Franky turns down the Mera Mera no Mi and encourages Luffy to take it as it'd be his only chance:
His duel with Senior Pink is one of the most respectable moments in OP, and he even comments that they should have a drink in the future if they cross paths:
His speech about saving the Tontatta and Kyros was one of the best scenes in that entire arc.
Dude destroyed the fucking blueprints to an Ancient Weapon knowing the destruction it could cause in the wrong hands.
Franky finding Caribou in the barrel on the Thousand Sunny was one of the most peak comedy moments in the history of OP.
He's the only one in the crew who technically met Oden, and he even turned down Oden's offer to put in a good word with the Pirate King, Roger. Franky could have technically already gone to Laugh Tale if he'd taken this offer:
I'd argue that him and Brook are the best post-TS characters in terms of how useful they've been to the crew relative to their actual strength level.
Bonus:
Franky changed the SMILE Factory manager's heart with a simple kiss. One of the best scenes in Dressrosa. The hard-boiled fight is one of the most awesome scenes in the anime.
His defeat of Senior Pink was absolutely epic, and his post-battle dialogue is close to Luffy's final scene with Katakuri. Oda took a largely unpopular character at the time (Senior Pink) and gave him a back-story that made him empathetic and likeable, as well as a personality that showed respect and honor in a fight. He also had Franky's energy mirror this so well. One of my favorite scenes in OP.
Look at how similar the ending is to Luffy vs Katakuri (animated):
Franky
Luffy
It's a crime that he's not more popular.