You may already know this, but there is no “best” airsoft pistol. Just like with real guns, the gunner is always more important than the gun. An experienced person will do fine with just a basic weapon, and people will always have their own personal preferences. But having said that, of course it is fine to buy a better gun.
If you are going to get a sub $100 gun then you will definitely want to get a gas powered gun. Why? Because electric pistols and springers do not have the same power as a gas gun, and usually are not built as well so they will not be as sturdy.
A good gas gun should have mostly metal parts, which will both help it to last longer and to add some weight to it making it look and feel more realistic. It will also be more powerful than a cheap springer, and unlike springers you won’t have to cock the gun before each shot.
There are many options available, so you should take a look around and choose the one you want, but here is one example of a good gun to get that costs just a little under $100.
The WE Full Metal 1911 Single Stack is about $95, making it cheaper than many other gas powered guns, but still a very popular choice. It can run on green gas, red gas, or propane and shoots up to 330 feet per second. It is full metal which makes it look and feel like a real 1911. Also, since it is a gas blowback pistol it has a realistic blowback action whenever you fire a shot.
TSD Tactical Semi-Auto M9 full metal gas blowback airsoft pistol is another great choice for under $100. It comes with a 26 round gas powered magazine, that can shoot bb’s 300 fps and is also accurate. You can buy a cheaper less powerful plastic version, but I recommend the metal version because it is more durable. I have been personally using this gun for the last 2 years, and I haven’t had any problems with it.
For less than $100 this is an excellent choice for a pistol, especially if you are interested in using a pistol as a primary weapon instead of just a backup in emergency situations. Remember that there are many other options available in this same price range that are much better than any spring guns you may be using right now.