Do a medicine sub-I -- it isn't fun, but what doesn't kill you makes you stronger. Radiology PDs like to see solid clinical groundwork, and you'll be more attractive for a sweet TY or prelim year.
Otherwise... take whatever interests you. I have an interest in path, so I took surg path and derm path electives. Anesthesia is a good rotation, as you'll get lots of practice getting tough IVs and placing central/arterial lines. Some people prefer a surgical ICU rotation to get those skills.
If you can spend some of your elective time doing rads research, do it! You'll gain a different perspective on radiology, and it may expose an interest in academics. If nothing else, showing intellectual curiosity will serve you well when someone is reading your ERAS application.
I've heard from lots of rad applicants that a med or surg sub-I isn't really worth it, but you're saying otherwise. I've been wondering if I should do one, but I won't be able to do it until after my app gets in anyway, so I don't know if it'll make a difference, unless it's on my list of scheduled classes.