Probably Donquixote Doflamingo
But knowing what you truly mean... I wager it is either Diamante or Pica. I'm leaning towards Pica.
Firstly, Doflamingo has a bit of a card theme: Spades, Hearts, Diamonds, Clubs.
The "Seats" seemingly represent Doflamingo's strongest. Especially since Doflamingo wanted to add Law as the Heart. Now let's look at a deck of cards... We have four Aces: Ace of Spades, Diamonds, Clubs, and Hearts.
Diamante: Diamond Seat, name reflects it.
Pica: Spades, Spanish for spade as I recall (his name).
Trebol: Clubs, I believe his name is like Pica's.
Law: hearts (he notably takes people's hearts...more importantly the Kora-San he mentions has a name that reflects heart in Spanish).
Seemingly they'd be your equal forces, except Law went rebel leaving the top 3 as Pica, Diamante, and Trebol.
In a deck of cards, have you ever noticed that the Ace of Spades always has a unique design? Despite being like the other aces, it is generally awarded the individual attention. Let's also analyze where everyone is.
Diamante: he's in charge of the Colosium. However, despite looking like that is the most important spot...think of this. He only expects Diamante to fight people who've fought others. While Doflamingo values his strength, he also doesn't want him fighting these people at full strength.
Trebol: Trebol isn't on the front lines. Instead, he's guarding Sugar in a low knowledge location. Meaning chances of finding and fighting him are low. He's likely the weakest.
Pica: Pica's location isn't conformed, but he was never ordered to leave the palace. Furthermore, Baby-5 and Gladius who are Spades Soldiers, are at the Palace. It's fair to assume he's here.
So we have Pica, with the unique Ace as his suite (spades), with Zoro (who typically fights the number two of the enemy crew) heading towards the palace where he's seemingly at, and whose the only one that hasn't been revealed.
Also, I'm sure you've notices, but this arc neatly parallels the Alabasta arc.
Mr. 3: Kind of goofy, oozing candle wax ==> Trebol: kinda goofy, oozing snot
Mr. 2: Eccentric attitude, leg fighter ==> Diamante: lively attitude, long leg tribe
Mr. 1: serious, seldom spoken, last revealed ==> Pica: seemingly serious, has said nothing, last revealed.
All in all, I'm feeling its Pica the Ace of Spades.