Let me start off by saying that Glock pistols are not in a class by themselves. The difference between being armed and being unarmed is dramatic; the differences between brands of guns are subtle and debateable. If you buy a gun from a reputable manufacturer, it feels good in your hand, and you understand and have confidence in its operating and safety systems, then the gun you have is good enough and you should turn your attention to becoming a better shooter.
Let me also say that any discussion of which gun is best must start with the use to which the gun will be put. In my opinion, self-loading pistols in 9mm or above are mainly suited to self defense, and particularly to concealed carry. A secondary purpose is for competition in IPSC or other sports which seek to simulate shooting in self defense.
That being said, I own a Glock-19 (compact 9mm) and I consider it to be the best gun for me. This is because the Glock "safe-action" trigger system allows me to shoot faster and straighter than I have been able to shoot with other self-loading pistols which I have owned in the past, while at the same time I am confident that I will not have a negligent discharge. Conventional "double-action" self-loaders, such as the SIGs, have a long, hard trigger pull on the first shot, and I found that, even with lots and lots of practice, I could not hit as fast with the first shot as I could with subsequent shots. Conventional "single-action" self-loaders, such as the Colt Government Model, require too much attention to operating the external safety lever - I am not 100% confident that I would always do this correctly, particularly under stress.
The Glock, on the other hand, has a relatively short, light trigger pull that I find I can manage well. The trigger pull is the same on every shot, and whenever my finger is off the trigger, the pistol is drop-safe. There are no external safety controls to manipulate (or forget to manipulate). The Glock's normal mode of operation is equivalent to carrying a cocked and unlocked single action pistol - which would be highly unsafe with any pistol except the Glock.
Glock pistols have a number of secondary advantages - they are among the smallest and lightest self-loaders in 9mm or .40SW. The 10mm and .45ACP models are somewhat larger but still quite light. They are very smooth and snag-free, a definite plus for concealed carry. They fit double-column magazines into a grip size that usually requires the magazine to be single-column. They are tremendously durable: Glock Inc has a Model-17 which, as of March 1994, had fired 347,000 rounds of 9mm +P+ ammo. The slide and barrel are surface hardened like a file, while most of the other metal parts are nickel plated. They have a low bore axis which makes recoil somewhat easier to manage than in hammer-fired pistols of the same weight. The have relatively few parts and are very easy to disassemble and maintain. Lastly, Glock gives you all of this at an attractive price: $420 new, retail for any of the 9mm or .40 models, as of April, 1994 in Seattle, WA.
In all fairness I should also mention two disadvantages of the Glock - these are light guns and therefore they recoil more than a heavier, all steel gun would with the same ammunition. In the heavier calibers, especially, recoil can take some getting used to, although in 9mm I find it quite tolerable. Second, the grip is a single piece of injection molded plastic. This means that grip shape and size cannot be modified with aftermarket screw-on panels, as with other pistols. If the stock Glock grip feels good in your hand, great. If it doesn't, very little can be done except to buy another brand of pistol. Recently, Hogue has come out with a rubber "grip sock" which slips over the Glock grip, giving it a soft rubber surface and a slightly more rounded shape. Many shooters seem to like these, but I do not, mainly because soft rubber grips tend to grab on cloth and thus make the gun less concealable.
Let me close by repeating the great advantage of the Glock against all other self-loading pistols: the "safe-action" trigger system provides the fastest accurate first shot on the market, and the first shot is the one that wins gunfights. If you want a pistol for self protection, this alone should make you look seriously at the Glock.