I'm torn between two lenses for this thread...
First, the HD DA21. I expected it to be excellent (and it is), but it's wider than I'd typically shoot. Most of my APS-C photography falls naturally between 28mm and 80mm focal lengths, and 21mm is considerably outside that range - so I didn't expect to use it all that often. But this little lens really seems to give me a creative boost. I'm rarely the kind of fella who shoots with a single prime lens for an entire day, but I think I could do it with the DA21. It's one of my favourites, and by far my favourite DA Limited.
Second, and far more recently, the Sigma 30mm f/1.4 Art. 30mm is a perfect focal length for me on APS-C, towards the wider end of my most-used focal length range. I wanted something fast, but couldn't justify the expense of an FA31. I'd considered the Sigma for quite some time but was put off by many reports of poor AF accuracy and dreadful CA / fringing. Finally, about six months ago, I bought one at a price that was too good to pass up... and I've been wowed by it. The build quality is lovely, AF on my K-3 and K-3II is reliably accurate, and the rendering is - to me - simply beautiful... almost like a modern interpretation of my favourite Soviet lenses. Yes, there's plenty of lateral CA and fringing, but the worst of it is easy to clean up in post-processing, and I can live with that in return for the overall look of images it produces. My only regret is that I didn't buy it sooner
I guess if I have to choose just one lens that surprised me the most, it's the Sigma - because for me it's so much better than the reviews led me to expect