Ahh tough question indeed. I cannot decide between my favorite Rockman characters.
X - He is a quick and powerful badass with caring human emotions, a strong sense of justice, and, as Mr. Acrosurge stated, inspiring leadership (the proof is in how he created Neo Arcadia for both humans and reploids (until Copy X took over and screwed it up)). He always worried for others and that, in turn, is the secret to his power. The 'X' in his name represents infinite power and infinite possibilities. He can obtain weapons from irregulars and strong powers in the armors from capsules (that and he's the first RM character I've ever played with so yeah).
Zero - Dude the guy is just awesome. He wields a freaking sword! He sacrifices himself for his friends and no matter how many times he's "died" he always comes back stronger than before! He is calm, collected, and always follows orders (pure loyalty man
). Instead of getting weapons from irregulars he learns their battle techniques. Also he is basically the first maverick; no matter what's in his past he never lets it get to him, in fact he destroyed Omega (original Zero body with maximized powers) proving that he is the one and only Zero (that can kick your ass with or without his awesome power).
Maybe one more than the other at times but I can never choose between those two. -^^-"