People who are verifiable one man armies like Tormund, people who have fought and downed one of the most dangerous and the strongest knights in Westeros like Oberyn, people who have demonstrated superhuman strength like Gregor (I'm not sure what the OP means by "best", if they mean most skilled this isn't relevant, if they mean the most effective it is), etc.
Brienne's feats just aren't particularly impressive. That it took her so long to beat a man who was starved, out of practice, and chained looks much less impressive than Eddard arguably getting the better of that same man while he's in his prime in a duel. Oh, she also fought Sandor Clegane with a Valyrian steel sword. The deck is always stacked in her favor.
Though actually, Ramsay is the best fighter in the show.
Who else can slaughter heavily armed and armoured Ironborn warriors with just two daggers and completely shirtless?
iirc, GRRM has said that Arthur Dayne and Barristan Selmy are the two best fighters (he might have said "swordsmen"), but Dayne would have a slight advantage if he was using his meteor-sword should they have dueled
The fat man said that a fight between Dayne and Selmy in their primes would be a stalemate, with Dayne's sword tipping the scales, but he never said they were the two best fighters.
IIRC he once listed a few people he considered the overall best fighters in the show (I think he compared them to professional athletes compared to their peers). IIRC Jaime (with hand), Gregor, Sandor, Barristan, Oberyn and Loras made the list.