This is my perspective, 70 days in on Global's Legends:
Tier 0:
Log Luffy: the best Rainbow captain in the game
Doflamingo: Orb Boost + Matching anytime anywhere; he can make or break clearing lots of content singlehandedly, or make hard content far easier
Marco: Forest God
Sengoku: the speedrunner's best friend, speed is its own reward
Borsalino: Penapple
Tier 1:
Fujitora: I Kill Everything
SW Ace: Ol Reliable
Crocodile: Ol Reliable's more RR-heavy older brother
Cavendish: Die Coliseum, die! Early game player's BFF
Mihawk: I am just good in every sense of the word, except for my special which is excellent
Whitebeard: Slow on the startup, downright brutal to the finish line...and arguably a better Striker lead than Barto
Rayleigh: I'm the most flexible captain in the world...stay thirsty my friends
Tier 2:
Jinbe: Easier on the startup for a new account, but loses the race to the finish compared to Ace and Croc
Corazon: I was made to kill one thing, and I do that very, very well
Bartolomeo: I'm good all around, but very RR-dependent to hang with Tier 1 boys
Lucci: Good lead, but the rest of the Powerhouse roster leaves something to be desired
Shanks: Decent captain ability, baaaaaad special
Tier 3:
Kuzan: Bitch I'm a monster, no good stage crusher, but look who's in trouble, I'm a pain in the ass to use no matter how great I am
Law: Also a pain in the ass to use, and is put to shame for each of his classes by other Legends.
Sabo: Pain in the ass to use effectively, not a particularly good Legend either, and his Raid Boss puts him to shame
Hancock: Female. Marco. But. Worse. Also what is a clear rate