Naruto should be the obvious choice.
He constantly pursued someone who:
Tried to kill Him 4 times.
Tried to kill his original crush, Sakura.
Tried to kill one of his mentors, Bee.
Tried to kill his sensei, Kakashi.
Whose mentor (Orochimaru) tried to kill his mentor, Jiraiya.
Who joined the akatsuki, and went around kidnapping people (bee) and terrorizing villages (cloud).
Whose brother (Itachi) is a dangerous criminal that slaughtered his entire clan and tried to kill Naruto, Kakashi, Jiraiya, and Sasuke (all whom he holds dearly).
Who killed his own brother.
Who killed his own Mentor.
Who betrayed almost everyone he's ever came into contact with at least once.
ANYONE who has the balls to pursue someone after they've done all that sh*t to discourage you not to pursue them, then they're unmatched in regards to obsession.