Well, a STOP would certainly be interesting if one could be found, but not a very practical gun; they are fairly fragile and parts are almost unobtainable.
In the pocket pistol area, a Colt .32 Model 1903 or .380 Model 1908 would have collector value and also be reasonably useful. The same would be true of the original German Walther PP or PPK. An S&W Model 19 would be a good choice as a very desirable gun and also a practical one. And I don't think a good .22 is ever out of place, with a K-22, a Ruger Single Six or a Ruger or Browning auto being good choices.