By far the Zero series.
It had the best music in the entire Megaman franchise in my opinion (Megaman Zero's 2 Introduction stage music... Departure for the win). And the most epic boss battle in the entire franchise as well. The boss battle was so popular, it was even an easter egg in Megaman ZX and you could do it again. (Trying to avoid spoilers)
Aside from that, the game play was by far the most fun. I like having plenty of choice on how I want to attack a boss battle other then just shooting it.. ZX had this style too but it was extremely limited to only transformations. Megaman Zero had the Ex Skill system which allowed you to not have to pause and transform to conduct more then two or three styles of different attacks; you had pretty much 10 or so ways to attack an enemy with just having two weapons equipped. This opened up a heck of a lot more gameplay style, which later in the series evolved further with equipable elves and elemental abilities, changeable forms, and chips that could be equipped in your head, body, and feet slot, and the choice of weapons you actually engage the enemies with. In my opinion, the Megaman Zero series had the best freedom in gameplay styles for every player. There's so much choice on how to build play and build Zero with all the power ups you can find and obtain.
My only complaint is that to get even half of these choices, you have to be at least better then the average player in the first place. The series really punishes you if you suck since every single mistake you do in a mission will be recorded in a score and will deduct you; even if you take too long to complete a mission or use elves in the missions in the first place (The exception of this being in Megaman Zero 3 and 4 since there were equippable elves that did not penalize you but still limited you. This made elves actually useful on a first playthrough)
TLDR: The Zero series in my opinion had the best music, epic storyline, and the best gameplay ever in the entire Megaman franchise for being extremely open to almost all types of people's play styles.
By far the Zero series.
It had the best music in the entire Megaman franchise in my opinion (Megaman Zero's 2 Introduction stage music... Departure for the win). And the most epic boss battle in the entire franchise as well. The boss battle was so popular, it was even an easter egg in Megaman ZX and you could do it again. (Trying to avoid spoilers)
Aside from that, the game play was by far the most fun. I like having plenty of choice on how I want to attack a boss battle other then just shooting it.. ZX had this style too but it was extremely limited to only transformations. Megaman Zero had the Ex Skill system which allowed you to not have to pause and transform to conduct more then two or three styles of different attacks; you had pretty much 10 or so ways to attack an enemy with just having two weapons equipped. This opened up a heck of a lot more gameplay style, which later in the series evolved further with equipable elves and elemental abilities, changeable forms, and chips that could be equipped in your head, body, and feet slot, and the choice of weapons you actually engage the enemies with. In my opinion, the Megaman Zero series had the best freedom in gameplay styles for every player. There's so much choice on how to build play and build Zero with all the power ups you can find and obtain.
My only complaint is that to get even half of these choices, you have to be at least better then the average player in the first place. The series really punishes you if you suck since every single mistake you do in a mission will be recorded in a score and will deduct you; even if you take too long to complete a mission or use elves in the missions in the first place (The exception of this being in Megaman Zero 3 and 4 since there were equippable elves that did not penalize you but still limited you. This made elves actually useful on a first playthrough)
TLDR: The Zero series in my opinion had the best music, epic storyline, and the best gameplay ever in the entire Megaman franchise for being extremely open to almost all types of people's play styles.