People often want to know the answer to this question, but really, the best response is whatever suits you best.
Consider your budget, your needs, the size of your hands, what you feel comfortable with, etc.
It's best to choose a firearm of a common defense caliber. Those calibers are 9x19mm Para, .357 SIG, .40 S&W, and .45 ACP.
If you live in California or Massachusetts, you will be restricted to a maximum of a 10-round magazine.
9mm ammunition will have the largest magazine capacities, .357 SIG and .40 S&W in the middle, and .45 ACP with the smallest magazine capacities.
.45 ACP has the most recoil and smallest magazine, but it also has the most stopping power of these calibers.
If you're on a tight budget (roughly $400), a Ruger might be right for you. Ruger's P90 is a reasonably reliable weapon that holds 7 or 8 rounds.
If you want to spend about $100 more, then you might prefer a Glock. Glocks are lightweight and have large magazine capacities. The Glock 17 (9mm) holds 17 rounds, and the Glock 22 (.40 S&W) holds 15 rounds. The FBI issues the Glock 22 to special agents.
Another reliable weapon in this price range is the Beretta 92FS, issued to military servicemen and many police officers (Mel Gibson's gun in the Lethal Weapon series), it fires 15 9mm rounds.
If you have a much larger budget to spend $700-$900, many people prefer SIG Sauer or H&K weapons. You can buy a SIG P226 chambered in 9mm, .40 S&W, or .357 SIG, and you can buy a Heckler & Koch USP in 9mm, 40 S&W, or .45 ACP.
Note that these are all full-sized pistols. If you're looking for a compact pistol for concealed carry, there is the Glock 26, Walther P99C, Beretta Px4SC, and H&K 2000SK. These guns can be concealed easily, but they are sometimes too small for people to grip really well.
I hope this information helps. Really, the firearm that is right for you is whatever you can afford and feel comfortable with.
If you're a lady or if you have a wife, she may prefer something in a smaller caliber, typically .380 ACP.
If you want something specifically for home defense and nothing else, you might prefer a shotgun. The Mossberg 590 costs under $400 and holds 9 12-Gauge shells. Remington also makes the 870 line of shotguns, which are more expensive and hold fewer rounds, but are largely considered to be more reliable.
Important note: I did not realize you lived in Washington, D.C. You should be aware that possessing an assembled pistol in D.C. is illegal. There is a case appealing D.C.'s law before the U.S. Supreme Court right now, and the Court is expected to rule on the constitutionality of the law before the end of June.