I have had quite a bit of shooting time with all three (though the P99 I have is a compact model). I can't see any reason to go with the Walther despite it being a reliable and accurate gun. It's just another striker fired polymer framed gun, and if you want something non-Glock for some reason, take an M&P or even an XD. Just do it for reasonably priced mags and more aftermarket parts if nothing else.
Now the Sig, I personally hate for the same reason as Jose -- the slide release is in the wrong place if you shoot thumbs forward. And, I really don't like DA/SA much these days. But having said all of that, if you really like the Sig then it's a good gun, well built, and will run just fine. Some people complain about Sig quality in recent years, but the guns I have seen run fine. I still have a Sig P220 (.45acp) and I like it, even despite the ergonomics issues I have with the design. I'll probably sell it eventually because I think the M&P .45 is a better gun for less money, but I can't knock it if you like the design.
Finally, the Glock 19 really is as good as people say, mags are cheap, accuracy is solid, the gun runs without issue (don't know about gen 4s, however), and is remarkably light and compact for what it is. It's my daily carry gun.