My GP100 SS 4.2" was my first revolver, and it will always have a special spot in my safe. I've put hundreds of rounds down it (maybe thousands at this point), and I always enjoy it. The trigger has really smoothed out with use, and I also spent quite a few hours smoothing out the sharp edges on the trigger, trigger guard, and hammer with a diamond file, and then polishing it up, so it didn't cut me up when I was shooting it. The finish is not as nice as the Smiths, nor is the trigger, but it is still fun. I put wooden Hogue finger groove grips on mine.
Having said all that, I much prefer my Models 686, 66, 19, 27-2, Python, King Cobra.....They are all 6" of course, but I did have a 6" GP100 for a while as well and traded it. I just like the overall fit, finish, and trigger that much better on the S&W (and won't mention the Colts as they weren't part of OP's options). Of them all, I like the K frame 19/66 more so than the 686 with full lug. The balance and feel works for me. If I could only have one, it would be a M66.