If you want to be a clinical instructor ... then NICU would not be a good choice for you. Undergraduate schools
don't hire faculty members to teach undergraduate NICU clinicals. They hire "pediatrics instructors" or "med-surg instructors" or "community health instructors" or "psych instructors" -- but not NICU instructors. I say this as someone who specialized in NICU and who has a PhD in nursing.
As a PICU nurse, you could get hired to teach pediatric rotations -- but without general care pediatrics experience, you would be at a disadvantage. Just because you are a good PICU nurse (providing care for 1 or 2 patients) doesn't mean you have much expertise with the general care inpatient peds population.
If getting a job as a clinical instructor is your goal, you will need to develop clinical expertise in a specialty where students go for full rotations on a regular basis -- not some sub-specialty that is not emphasized in nursing school
. Think about the major rotations that most students do. Adult ICU would give you more teaching opportunities. (But adult med-surg and/or geriatrics would probably give you the most career options.)
The 3 types of ICU's you mention are so dramatically different from each other that I believe you really need to think a little more about your likes/dislikes and preferences before you committ yourself. Do you really want to work with peds patients: Can you handle it when they die? Do you want to work with trauma victims? Child abuse victims? Do you want to work with post-op patients or adult medicine patients? Do you want to work with neonates? Think about the family dynamics you will have to deal with in each type of unit, etc. Which appeals to you most?