Kingsley would win this, he's portrayed as one of the most powerful wizards in the book, and by book 7 he was able to fight his way out when he was trapped by a group of Death Eaters.
And btw, Sirius was being reckless when fighting with Bellatrix, there was a particular emphasis put on the fact that his year or so pent up in Grimmauld place had made him reckless. So when he finally got into a fight that he longed for he foolishly tried to toy with Bellatrix and payed for it.
I'd say Flitwick, together they beat off Snape, and later McGonagall fought Voldemort with others and lost, while Flitwick took out Dolohov, who is very strong and even took out Lupin. You could say that only the best would take on Voldemort, but maybe Flitwick tried to but got held up by Dolohov, while McGonagall's path to Voldemort was clear. And Kingsley took on Voldemort, and I wouldn't put him very far ahead of Lupin, who Flitwick is ahead of. Plus, Flitwick was once a dueling champion, McGonagall never got any awards.
Clearly someone need to be schooled in the art of English Literature. Now look at this quote real carefully:
"Voldemort's fury at the fall of his last, best lieutenant exploded with the force of a bomb!"
Given the context of the quote, where it's directly speaking in reference to Voldemort's fury, it's clear that the 'last, best lieutenant' was an indirect form of voicing Voldemort's thoughts on the matter.
Guess what? VOLDEMORT DIDN'T KNOW THAT SNAPE HAD BEEN WORKING WITH DUMBLEDORE AT THIS POINT! Ergo, I'm right, your attempt at a rebuttal fails, and you lose.
Dude, he was agreeing with you. You said Snape wasn't the best, he said Snape wasn't the best. Still, Coldy only calls Bella the best and last at the evry end when Snape is already dead, so he doesn't count. He didn't say "best ever" he just said "best" and even if he had meant best ever, of course he would consider her the best ever cause Snape was against Voldemort! Either way it doesn't mean that bellatrix is stronger than Snape.