I miss my Olympus 11-22.
It was an odd lens, with a focal length equivalent to 22-44mm on a full-frame camera. Usefully wider than a 28- or even 24mm-to-whatever lens, it stretched out to a standard view with excellent quality and a reasonable size. I sold it when I foolishly though that the Olympus 12-60 could replace both it and the 14-54; I fundamentally did not understand what was important to me. That mistake was the beginning of the end for my Olympus system. It's the only lens that I miss even after all these years.
Part of my regret is that this was the first time I had a perfect pair: coupled with the 50/2 macro it was a small and unbeatable combination. I kept the 50/2 and Olympus E-1, and if I still had the 11-22 then maybe I'd be thinking seriously about the E-M1 (or its successor) instead of shifting to compact fixed-lens cameras.
But the lesson of that first perfect set has stayed with me even though I learned it too late. The Sigma 35/1.4 would be far less useful to me if I didn't have Nikon's 60mm Macro, and the wide Ricoh GR gets along very well with my Sigma DP3M. Having the two together is vastly better than one alone. Thanks to what I learned from losing the 11-22 I now look for opportunities like these to create synergistic combinations.