The .380 ACP (or 9x17 for the Euros) is the minimal caliber considered adequate for defensive purposes.  There are many good .380 pistols availble at reasonable prices.

 

One of the overlooked guns in this class is the 9.2x18 Makarov pistol.  The Mak is available in either its original caliber or, some were rebarreled to .380 ACP.  The Makarov is very similar to the Walther PP pistol in operation.  The only difference between them is the Mak does not have the Walther's loaded chamber indicator and the safety on the Mak is opposite to the Walther.  On a Mak, when the safety is down the gun is set to FIRE and when it is up, the gun is on SAFE.  The Walther is just the opposite.

 

Of the Maks on the market, the East German and Russian guns are preferred.  There are some others made by former Warsaw Pact countries and by China.  The Chinese guns are not as well-finished as the E. German/Russian guns, but they work fine.  The Chinese guns (and some commercial Russian production) were imported in both .380 ACP and 9.2x18 Makarov.

 

If you're looking for a good gun in this class, consider a Makarov.  You get a lot of pistol for you money.