I'll expand this topic to include small-caliber mouseguns by nominating the .32ACP/.380ACP FN Browning M1910 and M1922 (aka 1910/22).
These guns are wonderfully slim and incredibly easy to conceal. They're accurate, they have short, crisp triggers, and due to the lack of a separate guide rod- the recoil spring surrounds the barrel- they have an extremely low bore axis that makes them ridiculously easy to shoot quickly. Compared to most other early 20th-century European small-caliber automatics, the design is really simple, they're rugged enough to shoot frequently with confidence, and the high production numbers make for low prices and good parts availability. (Spare mags don't cost $75!)
It's true that they lack some modern features- the slide doesn't lock back on an empty mag, the mag release is the Euro-style latch on the butt, there's a mag safety and grip safety yet no RH-side thumb safety, it's difficult to undo the M1910 barrel bushing without a tool, and the M1910 sights are so small that they're functionally almost nonexistent. (OTOH the gun's slender shape means you can aim it surprisingly well inside 10yds simply by looking down the barrel like you're aiming a shotgun.) Also, they're a SAO design with a thumb safety at a time when DAO mouseguns are stylish.
I've been surprised at how few shooters even know what these guns are. They seem to languish on gun-show tables, and I get more "What's That Thing?" questions when I take my M1922 to the range than with any other gun I own.
There's a reason why these get nominated each time there's a "What Out-of-Production Guns Should Be Brought Back?" thread.
