Just to agree here - my third letter was from Peds as well and I consistently got comments in interviews about how glowing the letter was, even at interviews in the top 10 programs. Some interviewers even read me portions of it to comment on how much of an endorsement I got from that letter. I chose Peds not because I particularly liked Pediatrics more than Medicine or Surgery but because my attending in Peds was very very personable, very engaged in teaching me, and gave me good, honest feedback during my rotation. I got along with that attending very well and I felt like they knew me personally the best of all my core rotation attendings. I'd give the advice to pick your core rotation letter not based on which rotation it was (med/surg) but based on which attending you connected best with because they are more likely to write you a strong, personal letter.