Perhaps in order, but maybe not:
1. DA* 300 — I thought I was insane dropping $1,100 on this lens back in 2010, but it was the best photography purchase I've ever made. I have pictures from this lens riding around on buses in town (for adds), and posters draped on lamposts in front of the local zoo.
2. FA 43 - the best "normal" lens I have ever used (and I own a copy of the K 50/1.2). Exquisite color + excellent contrast + precise but smooth rendering of detail = lens with great character.
3. DA 21 - all-time favorite APS-C lens. Wonderfully compact, sharp, contrasty, great color, beautiful rendering — what more could be desired, realistically, from a pancake lens?
4. F 17-28: the original fish-eye zoom, not necessarily the best lens for resolving very fine detail, but what color, what rendering, what imaging capabilities it features. Last summer I spent a morning in Many Glacier in Glacier NP, shooting this lens, the DFA 28-105, the DFA 15-30, and the DA 35 Ltd and this lens produced the most beautiful images.
5. DFA 15-30 - from a purely technical standpoint, the best zoom lens I've ever owned. Sharp and very contrasty, with great color. Rather on the heavy side, but the agony of carrying it around builds character.