I'd suggest doing a rotation that gets you some inpatient experience with the specialty. The reason for this is that most specialty rotations are focused around the clinic, and if you never get out of a specialty's clinic, nobody will ever know you were there. If you can get out on the floors, you will get a chance to meet other attendings and some of the residents, and simply being recognizable and a "known quantity" is often a very important aspect to getting better consideration for a residency. You may have to do some research to find out which specialties are a busier at which institutions, because it may not be the same everywhere. For example, my medical school had one of the busier and better-known CF programs in the country, meaning Pulm pretty much always had kids in-house, whereas where I'm now doing my residency, one of the Pulm attendings occasionally pretty much consults herself on patients out of boredom.