Always - S&W Model 60 in .38 Spl.
In addition, I frequently carry a Springfield Champion in .45 ACP (Commander clone), a Colt Officer's ACP in .45 ACP, or a 2.5" Python (less frequently due to concerns about it going into a "rust locker," should I need to deploy and use it). I recently purchased a new Colt Lightweight Government Model in .45 ACP, and I foresee it pulling quite a bit of duty once I have had a chance to break it in. I sometimes carry other guns, including a 4" 19-2 and a 4" 15-3, but I have retired my 4" Python and 6" custom tuned Python permanently from defensive carry duty, again due to concerns about their treatment should they go to an evidence locker in case of a shooting, however unlikely that may be.