Most of the ones I see today are offered in .380 ACP, but James Bond carried it in .25 ACP, I believe.
If memory serves (which might be a first), Bond originally carried a .25 - possibly a Baby Browning until somebody reminded Ian Fleming just how big a joke that was. He switched to the PPK, although I can't recall which caliber, after that.
Bond also carried the .25 in a "chamois" holster, which was just plain silly.
I've got a pre-1968 German-made PPK/S (no thumb rests to satisfy GCA68) that is rather unpleasant to shoot, and kind of heavy. I'd vote for a compact nine if I had to do that over again. But it's solid, and reliable. My range buddy just bought a new S&W version, and I prefer it, although it needs to be dehorned (sharp edges smoothed off, etc.).
I went to my dealer's the other month to look for a compact 9mm for myself and ended up with a second-hand S&W CS45. Almost the same weight, size, and form factor as a PPK/S, but about twice as fat. FWIW, it's also nearly as comfortable to shoot as my Commander, which surprised the heck out of me.
The little Smith is an excellent all-round carry, IMHO, but not a range gun, although it's quite accurate. I can still grab the PPK/S if I need something even smaller, or a couple of Commander sized guns for really serious carry.
Editorial: Don't join the "gun of the month club".... Always carry more or less in the same place, and at least be prepared to break the thumb-break if you're using
one even if your current holster doesn't have one. Try to keep the safety mechanism similar, too, and
practice as much as possible with everything you're likely to carry. Not just bullseye, too!