Depends on your goals. In Pulm, you'll see CF, asthma, and BPD, the latter two of which you will see plenty of during residency and in a general peds practice. In GI, you'll see lots and lots of constipation, plus some inflammatory bowel disease and short gut, plus some intermittent biliary atresia, fatty liver, etc. In Endo, you'll see a lot of diabetes, but will also see short stature, hypo- and hyper-thyroidism, septo-optic dysplasia, Turner's syndrome, precocious and delayed puberty, etc. You'll likely see more variety of pathology in Endo, but how much you see will depend on how big your center is.
I'm biased, but I think you'll get more out of an endo rotation than anything else if you are going into gen peds, just because growth is such a huge thing in pediatrics, and endo gets a good number of growth problems referred. Also, some adolescent medicine, not only with the kids with diabetes, but also all the puberty problems. But whether you'll get more out of it as an MS4 or as a resident, it's hard to say.