The final boss of Mega Man Zero 3 is Omega. Zero confronts this gigantic Reploid after Dr. Weil links Omega with the Dark Elf in an attempt to control Reploids worldwide.
Omega is especially significant because he is revealed to be using Zero’s original body. During Zero’s long hibernation, his memories and consciousness were transferred into a copy body, while Weil stole the original for Omega’s combat chassis. That revelation makes the battle personal as well as political.
The encounter is not the same as fighting Dr. Weil directly. Zero defeats Omega, and the Guardians help destroy his original body, killing him permanently. Omega’s death also releases the Dark Elf from Weil’s corruption. Dr. Weil remains the larger villain behind the crisis, but Omega is the game’s climactic boss battle and the single canonical answer to the question.
Copy X and Phantom are important characters from other points in the Zero storyline, while Dr. Weil is the antagonist orchestrating events rather than the final boss of this installment.