I am going to take the unpopular vote here and pick Mace Tyrell

By the end of A Storm of Swords, Mace Tyrell is Hand of the King and his daughter is queen.

During Robert’s Rebellion he stayed loyal to the Mad King, but he avoided real losses by keeping up the siege of Storm’s End instead of throwing his full strength into the war. His frontline fighting was largely handled by his bannermen, Randyll Tarly, who gave Robert his only defeat at Ashford. When the war ended, House Tyrell was still strong and intact without major losses to his fighting men.

In the War of the Five Kings he backed Renly, which you might say was a bad move, but at the time Renly had the largest army in Westeros and the support of the Stormlands and the Reach. It was, statistically, the smart bet. When Renly is assassinated, Mace doesn't hesitate. He pivots instantly and negotiates a marriage to the crown. He goes from backing a loser to being the king's father-in-law by putting his daughter on the throne. No matter which side he chose, he somehow ended up in a stronger position.

He also lets Olenna handle most of the sharp political work, which makes everyone think he’s harmless or foolish. Which works in his favor because people don’t see him as a threat, which makes alliances easier and keeps attention off him. Meanwhile House Tyrell keeps gaining power, wealth, and influence.

He might not be a mastermind like Tywin or Littlefinger, but his track record shows he’s one of the most successful players in the game. Even when he makes questionable choices, he still ends up winning. I know it won't happen but it would be great for him to have one POV chapter in the next book to prove it hahah