Rossi revolvers are usually a safe bet on a revolver for under $300. If you do some searching, you may even be able to find a used Smith under the limit.
As far as auto pistols go, Kel-tek makes a decent pistol that can be found new within your budget. If a larger frame is more to your liking, a used Ruger P series, or a Taurus 92 shouldnt be too hard to find under the $300 mark.
Sigmas are ugly, have a heavy trigger, but do the job, and are fairly light weight.
If you are one of those that just has to be different, look at some of the Soviet surplus pistols. Tokorevs, Makoravs, or any of the other variants that they produced are usually inexpensive, have enough power to be considered for personal defense, and usually are as reliable as they come.
Biggest thing is find something that fits YOU, and that you will be willing to (and can afford to) shoot enough to be effective.
I could not agree more. Ruger p series is a fine pistol. but i would suggest a Tokorev. 7.62x25mm hits with close to the same ballistic energy as a 357 magnum, ammo is cheaper than 9mm. and lets face it, a whole lot of dead nazis proves just how effective it is. a tt33 goes for well under 300 dollars. Good luck in your search.