Mostly agree with above.
Peds cardiology-great for listening to uncommon murmurs and the cardiovascular exam. To reinforce the above: a few years back adults with congenital heart disease overtook children with CHD in numbers-having some familiarity and comfort with this population could be an invaluable (and uncommon) asset for an internist. Lot of overlap in the electrophysiology world and you can brush up on EKG skills as well as broaden your scope of knowledge when it comes to channelopathies and cardiomyopathies. As a future internist I would try to do a rotation with a group that sees a lot of adult CHD.
Peds pulm-Just be aware that an incredible percentage of peds pulm is asthma, but that is a useful knowledge base. I find the field painful, so my bias is to not think it's the most useful.
Peds ID-I could see this one being very useful as there is plenty of overlap and we use the same drugs.
Peds Heme/Onc-you will see plenty of patient with very real disease. Though there is often a great difference in the epidemiology of childhood ca vs. adult, there are common principles (tumor lysis avoidance, for example). An outstanding benefit would be the awareness of how to take care of adult survivors of childhood cancers and the risks that those patients face.
Childe neuro-real disease and great reinforcement of the neuro exam. Similar concepts of treatment amongst similar disease, but some disease that won't be seen amongst adults.
Peds ED-dunno. I loved it as a resident, but for someone going into IM? There is so much non-acute stuff, it might be tedious for you.
Peds Endo could be interesting. Will get very comfy with DKA and general DM managment, but most will be type I (with an increasing proportion of type II)
Adolescent is good for routine gyn care in the young female population (DUB, contraception, pelvic exams, etc.) basic psych stuff (simple depression) and some sports med.
A/I is not peds specific, but is useful for any primary care field.
I do agree that PICU can give good experience with the stuff listed above. Now that ECMO is seen a lot more in the adult world, I don't know if it provides all that much to you though since you'll be doing lots of that in adult land anyway.
Not a peds rotation, but ENT sees a lot of kids in a lot of places. Learning a good otoscopic exam is really valuable and ENT clinic is definitely useful. Watching ENT surgeries can be painful, but, if you can arrange it, try to get the intubations in a T&A room. High turnover cases, so lots of opportunity to bag mask and intubate.