I've only about 100 TTMs over 20 years (I mostly go after the in-person). I've sent some out, though, for guys I could never get to...
Barry Sanders 1989 (rookie year)
Brett Hull (mid-90s)
John Glenn (only a couple years back)
But the most recent that I picked up was Betty White and she personalized it as I asked her to in my letter, which was pretty cool.