I own two of the three.
In .45 caliber, the Cougar is a joy. It rides very low in your hand and I find the recoil to be very mild for the caliber. I posted a side by side shooting comparison between the Cougar and a Sig P225 back in January and at that time, after several hundred rounds of side by side shooting, I thought the Cougar actually had less recoil than the Sig when it came to hot 9mm loads versus 45.
The grip on the Beretta, however, is a double stack and is substantially wider than either of the other two weapons that you mention. The trigger placement and the wonderful curve of the backstrap make the gun easy to handle for small hands, but the measurements are what they are--it's definitely wider than the Walther.
The slide racks back with the precise click-click of any Beretta and the finish is fabulous. The only thing to watch is that the sharpness of the decocker can bite you if you let the slide return to battery too quickly.
Putting out a post on the Firing Line asking for commentary on the PPK and the Sig 232 is like waving a red flag in front of a bull. I happen to love my little Walther and find it to be a reliable gun with FMJ's and Winchester Silvertips. It's a beautiful gun to behold in just about any configuration--aftermarket grips are so abundant you can practically choose to have a different grip to match every suit you own. You have to watch the way you hold the gun, a 'high hold' will result in being bitten by the blowback of the frame; however, my eleven year old son shoots my PPK at the range regularly and loves the gun.
The Sig fans (and I own two Sigs) are going to post that the US made Walther is a jammomatic that they gave away rather than hastle with, there hasn't been a good PPK made since they left Germany, the Sig is an improvement on the original German design, and then go on to claim that the 232 is the finest weapon made since the dawn of time.
The Kahr fans are going to post that the Sig is too wide and too big for the caliber and that you ought to go with a real gun like a K9 or a K40.
The Walther fans are going to claim that they've never had a problem with jamming or biting (I love my PPK, but it took 150 rounds of ammo to shake out the bugs in the feeding) and wonder what the questions are all about.
Ultimately, you're going to get alot of reccomendations to buy other weapons. But in response to your question, of the three you named, I would rather own the Cougar if it was going to be my only weapon. Easy to shoot, feels like an extension of your own hand, good caliber, great looks, and reliable...