Marco is the most powerful WB pirate besides WB himself. He was portrayed as the First Mate and fought all three admirals on even ground. He survived a hit from Garp and Kizaru's lasers, the latter without his regeneration ability. With Him being (possibly) the new captain of the remains of their crew is a testament to his strength and fighting capability which are easily Admiral-level.
Jozu was able to harm Aokiji for the first time in the series, and effortlessly tanked Mihawk's slash. He blitzed Crocodile with his speed and only lost to Aokiji due to a distraction. He is easily a high vice-admiral level fighter and can hold his own against the Admirals.
Vista parried with Mihawk and emerged unharmed, showing his own great skill in swordsmanship. He also emerged unharmed from fighting Akainu with the other WB commanders. Currently among the best swordsman in the series, besides Mihawk and Zoro, and a Haki user. Same capability as Jozu, only slightly less due to his lack of a fruit.
Ace was capable of defeating Blackbeard and his crew if he wasn't so reckless. He fought Jinbei and Aokiji, both times at a disadvantage. His usage of Haki is unknown, but with it, he had so much potential to become one of the most powerful characters in the series. At the time of his death, he was a vice-admiral/shichibukai level fighter.
Curiel fought Gekko Moriah and Akainu and managed to survive both encounters. It's possible he has Haki, and is a vice-admiral level fighter.