I wanted a Ruger SR22 for a while, but never got one. Recently, I came across a FEG of Hungary Mk-II AP22 imported by Interarms back in the late nineties. It's solid steel, very deeply blued, and fun to shoot. It's a rimfire version of FEG's popular PA-63, which comes in 9x18 Makarov (and I think in 9x17mm/.380ACP.) Because of this acquisition, I'm no longer looking at the Ruger. I do already own a Ruger Mk-II, and have had it since 1987.
But, of those on your list, the SR22 is the one I'd go with. The Walther and the SiG have had too much negative publicity for me, and the ISSC is (for me, at least) too "Glock-like" in appearance; Glock pistols just never seemed to "fit" my hands and grip.