Shinigami depend too much on innate aptitude- if you don't have a ton of spiritual power, you suuuuuck and are total fodder. Even if you have a little, it can be decades before you have significant power, and almost none get truly strong. The other two, even if you don't start out great, you can train up, especially if you find a good teacher (being born with some innate aptitude helps in both, but isn't totally required).
Even though most ninja have just one jutsu 'area,' those areas tend to be pretty flexible, and they have some miscellaneous abilities besides.
I'd aim to be a Fire/Water release type Better still, Boil release!
For ninja, I'd be purely physical strength, apart from techs to weaken my opponent.
Shinigami...Well, this isn't a shinigami, but that death looking guy was pretty op.
Pirate This is the fun world. I would probably chose a Zoan type, an eagle or falcon or something (great vision and really great at quick aerial attacks) And I'd like to combine this with the CP9 techs.
To be fair about One Piece, it's complete and utter, cliche', barely readable garbage, for the first couple dozen chapters.
It gets epic later.
"The Daemon lied with every breath. It could not help itself but to deceive and dismay, to riddle and ruin. The more we conversed, the closer I drew to one singularly ineluctable fact: I would gain no wisdom here."
Shinigami- balance between physical and kido, elemental zanpaku-to type. Pure kido types take too long to get strong, and physical shikai have too one-dimensional sword types, while elemental ones often have multiple abilities even in first release, let alone bankai. Maybe water.
Pirate/Marine. Obviously logia or one of the other uber fruid'd be most hax, but just picking something cool, I rather like Nico's fruit. She uses it to take out both small armies of foes at once, plus it can work on pretty tough individuals.