Freedom is there, so either that or Eternal Soul depending on the level of Freedom possessed. If it's Ultimate level Freedom (I don't think it would be absolute level), I'd 100% choose that. Other than that, Eternal Soul is a candidate for my ideal form of immortality, if I had to pick one, since it allows your soul to pass freely anywhere and construct physical forms from its limitless life force.
Absolute Reincarnation is also up there in terms of forms of immortality, since it seems you can somewhat customise your reincarnations and retain all previous capabilities, so you can decide the starting point of your new lives and basically accrue abilities over time. Though this isn't what this post is about.
