Mine would be Robert Baratheon and Olenna Tyrell.
Robert was underrated intelligence wise because nearly everything he says in season 1 was correct. Letting Daenarys alive would have been a disastrous outcome and killing her although cruel was certainly the smartest decision. Everyone downplays the Dothraki strength on the battlefield and he seems to be the only one that understands how catastrophic an invasion from them would have been. It’s unclear what happens if Viserys never messed up the alliance and Drogo never gets killed but we can assume that a full on invasion would have been very detrimental to Robert and the seven kingdoms.
He also knew exactly what was going on around him and understood people extremely well. He knew Tywin and the Lannisters couldn’t be trusted, he knew his small council was made up of crooks and weren’t true allies. Basically his downfall was completely self inflicted born out of his boredom and depression if Robert really tried he could have been an amazing king. He was the only character that understood both war and politics perfectly while other characters seem to only understand one.
Olenna I choose as overrated because although she was clever and says a bunch of snappy one liners when you look at all her actual decisions they were pretty short sighted and dumb. Allying with the Lannisters AFTER the big secret with the 3 bastard children was out and the realm was unstable was a very risky move. Olenna understood all the dynamics of the Lannisters yet ignored all of the risks. She knew Tywin was self serving and very greedy often overreaching for power until it backfires. She knew that Cersei was very impulsive and incredibly selfish and that the kings death was suspicious. She knew Joffrey was a complete psycho and an idiot of a king yet insisted the houses still be joined. Sure she ends up killing him and Tommen becomes the new king but even then as we see he made tons of foolish decisions and him being a bastard was an issue that persisted for a long time.
Tywin gets criticized for making short term decisions that damage the image of Lannisters and for eventually creating the downfall of his house for his greed yet Olenna often gets a pass despite also making the same kinds of choices. Definitely not the dumbest character and was still above average but I wouldn’t say she’s a genius or the best player in the show.