The 1911.
Just because there is so much you can do with it. It trumps every other handgun except in ammo capacity.
People say it's too heavy, but that's a good thing sometimes.
As for the Walther P99... It's an okay gun. I don't have any experience with the polymer Walther pistols. I have handled the P5 and P88 which are both similar to it. I know the old PPK/S when Walther manufactured them were great. And when they were in a rush to keep up with production numbers, the Smith & Wesson made ones were great. Ever since then, they've all been crap.
Otherwise I would really go with a Glock 19 if I wanted a polymer 9. Either that or a CZ P01.
For 1911s, I own a whole bunch of them. Two Springfield Mil-Specs. I sent one to the Custom Shop and had the Professional Package done for $1,800. Shoots like a dream. I have another Mil-Spec which is bone stock except for a deburring job. I have a TRP without the rail/no bull barrel. Good gun. Have a Loaded which is all built up for a target gun. Then I bought an EMP 9 last year which is a pretty good gun.
I have two Kimbers. A Tactical Custom II and Pro Carry II which are both all stock.
Have a Taurus which is all stock. Good gun for the money. A lot of people dis it cuz it's Brazlian but it shoots just as well as my Kimbers. It just uses a lot of MIM parts.
Had an RIA for awhile. Ended up selling it to buy the Taurus. It was a great gun for the money. Lot of MIM stuff.
Then I have two Colts. One is a 1960s Colt Government Model in .38 Super. The other is a Series 70 GCNM in .45 that I found at a Gander Mountain that was all worn out and I have put about $1,000 into it so far and it shoots great. All stock except I put a new barrel and the Series 80 style bushing in it.