Gosh. It's hard for me to say. I have the most fun with my Mark II. It's a great shooter and it makes me look like I know what I'm doing. If you have the target in those crosshairs, you are going to hit it. No ifs, ands or buts.
I love the daylights out of my Sig P238 for being a great pocket pistol. It's nice to look at and the trigger on it is wonderful. You can slip it into the Remora holster and put it in your pocket or IWB and you are good to go. (Well, not me. I live in Illinois.
My GP100 is the best HD/SD gun I have. If I am ever in fear for my life in my home, I guarantee you it's the .357 magnum revolver I'm going to reach for. It's the one that can take care of me the best.
I think they each are the the "Greatest" for the purpose they serve for me.