I'd say Jessica is out of the running, no offense to anyone who likes Jessica, I really like her, but come on, it's obvious. If something's happening she can't help but get involved, but she's reluctant to unless she get pulled into it.
The rest have character flaws. Danny is immature and there are things he runs away from dealing with. Luke can be cocky and stubborn and shit as shown in season 2 of Luke Cage
Spoiler:
As evidenced in his conversation with Claire where she got the **** out of there, and again at the end when he put himself in a precarious position as the self-appointed "sheriff"
. Matt repeatedly lies to his friends, albiet with the intent of keeping them safe but he certainly goes overboard with it. They also all have some violence and anger issues, Iron Fist going ham on the drunk dude and then attacking the other defenders and often losing his cool under pressure, Luke Cage beating the shit out of that dude and punching a hole in Claire's wall when she confronted him about her concerns, and Matt's violent and sadistic tendencies have already been mentioned.
For me what separates Matt from the others in regards to his character flaws is that he's much more willing to grapple with them and question himself. Those scenes in every season he appears in where he goes to confession are very important for demonstrating this fact. He struggles with the nature of what he does, actively seeks out guidance and counsel for parts of himself and moral conundrums he's struggling with, and repents and begs forgiveness for what he does that crosses a line. Luke
Spoiler:
had his girlfriend, someone close to him actively confronting him about his flaws and he wasn't taking her criticism seriously until he drove her off by punching a hole in her wall
, and Danny seems to lack the self-control to actually be more self-critical.
For that reason I'd consider Matt Murdock the most heroic defender.