Well, the thread title and the question in the OP are different. For example, Venus and Graf are remarkable fighters but rarely showed their emotions on court. Meanwhile, someone like Ivanovic looks far tougher than she is in spite of being a mediocre fighter.
Assuming you mean who was the least prone to fighting for a comeback, I'd go with Davenport. It is quite incongruous to me that she only has three slams, and it's worth noting that all three titles came without dropping a set. She needed to be playing well to win tournaments. As an aside, though, I think Davenport is severely underrated mentally. These are two different things. She wasn't a natural-born fighter, but she had to have incredible mental fortitude to go out and do her thing against players in spite of seeing shot after shot pass her by because of her movement. We criticize Davenport for going into the droopy mode so often, but in reality she stayed much more together on court than 99% of truly slow players throughout tennis. The ability to bounce back from opponents winners (and against elite players, I think Davenport was the most prone to a high opponent winner count) is very, very impressive mentally. But she wasn't a fighter.