Pistol
Old faithful
With its wonderful sniping capabilities and great stopping power, not to mention its large ammo clip, the pistol is a superb weapon that I hate to be without. When the enemy is at distance you can whittle them down with little or no risk. On Heroic you can drop an Elite inside six rapid shots about the head. Jackals and Grunts are down inside four shots, or just a single headshot (which is relatively easy with the Grunts). A Hunter goes down with a single shot to his vulnerable orange flesh.
Banshee buster
The pistol is also the best thing to take down a Banshee. On Heroic it takes something like 15 shots. This might sound unlikely, but bear in mind that the pistol fires high-explosive rounds; hence the high levels of damage it can do, presumably.
Speedy bullets
Another welcome aspect of the pistol is the shot speed. Because it's so high, you can hit distant laterally moving enemies without having to 'lead' the target, i.e. without having to aim off target in front of them (like you'd have to with a plasma rifle, if they're in range at all). The bullets get there almost instantaneously.
Automatic fire
The pistol goes into repeating automatic fire if you keep the trigger down, but personally I rarely use this, finding it harder to keep the aim adjusted. In any case you can achieve the same rate of fire manually. I sometimes use it when it's relatively easy to track the target, for example on a Banshee or on an Elite heading directly towards me.
Pistol heaven
If you're a pistol enthusiast like myself, let me urge you to get into the Far area defence in the rockslide megabattle. In that unique scenario you face numerous enemies closing in on you from various angles across an open plain, giving easily the best medium to long range pistol fun in the entire game. It's pistol heaven and a real test of your skills; not just of your marksmanship but also your ability to quickly weigh up and prioritize multiple incoming threats under pressure. Don't miss it! Meanwhile, for the best short to medium range pistol fun I recommend Death from on high, an option within the 'Plateau front defence' plan.