the single combat should be ranked based on the skill of the warriors, their personal rivalries and the impact their duel had on westeros. I would rank in the following order:
1. Bloodraven vs bittersteel:
they were brothers to begin with. Both were renowned as military commanders. They loved the same woman.
At the battle of the red grass field bloodraven decisively deployed his archers behind daemon blackfyre and killed him and his sons.
Bittersteel attacked this unit and routed them and defeated bloodraven in melee. Took an eye for a brother.
Though bloodraven lost the battle, he won the war by crippling the blackfyre cause.
2. Rhaegar vs Robert:
decided the fate of westeros. They were distant cousins. Both loved the same woman and fought for her hand.
The fate of the throne wrested on the balance as the silver prince and iron stag clashed beside the trident.
Rhaegar was one of the best swordsmen/jouster of the time as he was able to beat the likes of arthur dayne and selmy. Robert was beastly strong and the best mace fighter of the time.
As the battle waged in muddy ground and on horses, rhaegar's agility and skill with the sword was offset by robert's brutal strength.
Rhaegar's sword was only able to injure robert but robert's hammer crushed him.
3. The mountain and the viper:
another robert rhaegar style fight with the difference being that gregor was much more bigger and stronger and that the combat was on foot.
Gregor raped and killed oberyn's dear sister ensuring that the battle was fought for much more than just glory or poltics.
Oberyn miraculously defeated the mountain. But callously dropped his and guard and the mountain paid him generously with three armoured punches.
4. Barristan and maelys:
Both were legendary fighters. Barristan famously cut through the golden company and killed the monster. Thus ending the blackfyre's for good.
5. jon and halfhand: none of them were fighters of note. But the even set the stage for jon's story