As much as I want to say Abaddon or Ruby who are both very strong, brave fighters, I had to rewatch 5.19 “Hammer of the Gods” just to see if Kali, The Destroyer, could beat them and, I think it’s safe to say that, even as a one-off character, she kicked ass.
Not only did Kali, who is a deity or God in her own respect, stab Gabriel with his own angel blade just to prove to the rest of the Gods that Lucifer could be killed but she always kept fighting, holding her line and leading the other Gods, and there’s a reason that she was the only deity that made it out of that episode alive. Surprisingly to me, because she was a female character on a slightly misogynistic show, she is currently known to still be alive whereas Abaddon and Ruby are not. This may show that Kali is a better fighter than them but, bare in mind that, Gabriel does kind of come to her rescue before she could get killed by Lucifer like the other Gods.
However, Kali does bravely go head to head with Lucifer regardless of how much power he had over her, and manages to hold him off for a good while whilst Sam and Dean were cowering in the corner. I think she does a better job than the other Gods who Lucifer quickly and easily takes out on his way to her.