my experience has been that all pistols are in the same general accuracy range when you're talking about "combat pistols" at a range of less than 20 meters/yards or so. Most guns are also only as accurate as their handler, as well. The Walther P99 may be exactly what you want. I would suggest the AS model as it has a decocker, to allow safe carry with a DA pull followed by subsequent SA pulls, which will help with accuracy for follow up shots. The QA model, I can't really say anything about, but their DAO model is kind of clunky with the trigger pull, not that great. With the DAO, I believe they were trying to mimic the Glock style trigger/action and did a poor job at it.
I've got a suggestion, find a range that rents guns and rent every gun you would possible consider and try them out. When push comes to shove, it's really going to come down to what you like to shoot. I could be wrong about the P99, M&P and XD being for you, you many absolutely hate each one and end up getting something none of us have suggested yet. It's up to you, but discounting a gun because it's a certain make/model that one person told you about isn't the way to go about it.
The Walthers are all great guns (including the P22, so long as you take care of it), as are the Sigs (same goes for the mosquito as the P22), you can't go wrong with any S&W semis and I believe the Ruger semis would be too clunky and unfashionable for you. You might also like to take a look at the S&W Sigma and Ruger SR-9, though. They come in at a price much lower than most of the other guns we've mentioned, yet still sport many of the same features.
But it ultimately comes down to what you like, and not what other people tell you. Who knows? Maybe you'd be so comfortable with the grip and feel of the P99 that in your hands, it would outshoot a Sig in your hands. Same goes for the Glocks. And the XDs. And anything else you pickup. I can outshoot the M&P with a .45 1911 and a 9mm Sig, because I'm more comfortable with single stack grips and those specific designs than the M&P. You may have the opposite reaction.