Nami: Castform
This shouldn't be a no brainer. Castform is the literal weather Pokemon. Depending on the day of the weather, it can have three different forms. It's honestly the perfect Pokemon for Nami to have as her ace when sailing on the seas while navigating.
Ulti: Rampardos
This one is a no brainer as well. Ulti's main attacks revolves around headbutts since her Fruit "the Dragon Dragon Fruit, Model: Pachycephalosaurus." The Pachycephalosaurus is thought to believe have had head-butting competitions due to their large bony dome on its head which would've protected them from the vibrations from heat-butting similar to modern day rams or musk ox.
Big Mom: Slurpuff
Big Mom could honestly have quite a lot of potential aces similar to other characters. But, I honestly see Slurpuff being the one that fits Big Mom the most. Slurpuff's sense of smell is 100 times stronger than humans which is helpful when it comes to baking pastries or sweets. And like Big Mom, Slurpuff tends to enjoy sweets itself quite a lot. And kinda similar, but apparently Slurpuff can tell a person's mental and physical condition just by using taking in their smell which is kinda similar to how Big Mom can take a victims life force when they fear her.
Crocodile: Krookodile
This is another obvious one. Krookodile is a literal sand crocodile. It's first stage evolution is literally named "Sandile." But anyway, Krookodile fits Crocodile quite well. Being a sand crocodile with the name "crook" in it's name which Crocodile relates to since he's pirate (a bad one) and has lied to the entire nation of Alabasta that he was their supposed hero trying to frame and overthrow Cobra for his own personal goal which involved Pluton and overall power.