Kahr PM9, hands down. Easily the most comfortable mouse gun I have ever shot. There's something perfect (given the size/shape of my shooting hand) that just seems to match the PM9's back strap shape, width and recoil characteristics. Even with the hottest 9mm loads, it's pretty darned accurate as well.
Only downside, the ones I had never were reliable. Sold them. Sadly, too, 'cause IMO it's still the finest format on the market in that category (9mm mousegun that's got "big gun" characteristics of shootability, accuracy).