Subject could be "thank you." You can keep it short and sweet and email at least the PD. However, if it's a program you're legit interested in I'd put more effort into it, maybe personalize with tidbits from conversation or about the program in general, email everyone you interviewed with, etc. Some folks complain about this whole thank you email etiquette and what it has become but that's now just the way it is, and some programs do notice. An email can show interest and care by the applicant. Maybe it won't drastically change your rank list order, but programs also want residents who want to be there and will be happy at their program. Obviously don't lie or bs too heavily, but for your top programs, it won't hurt to show interest/enthusiasm.