IIRC, it was a Sigma 170-500. I already had a reasonable selection of lenses for use with my film bodies and wanted a long lens to complete the lineup from 12mm upwards. Still saving for the next step - a 300-800!!!
Going back a bit further, the second lens I got when I got back into photography in AF days was a 70-300 AF-G. Actually, that came as part of a 2 lens kit with the F65, along with a 28-70. Way before that, when I first got into the hobby, I'm pretty sure it was a faster 50 or 55 than I was given with the camera - a Pentax S1. After that, I think I got a 135mm for portraits and a little more reach than I already had.