Why would defeating “the freest man” be more heroic than defeating “the greatest evil”… I hope you aren’t implying I’m talking about power levels here
Because the embodiment of the pirate in OP is "the greatest evil".
Being the freest man, going by the literal meaning of it as a pirate, means that you're absolutely not held back by anything, not common sense, not principles, not crew, not even family. And none fits that description better than Teach. At least Imu is trying to maintain a balance of some kind.
If i had to choose which evil is greater, i would go with the guy that is literally darkness.