There are some other threads that pretty much cover everything ad naseum.
To avoid any confusion, the guns TPB mentioned are for the original consoles, not for emulators.
A lot of people come to the conclusion that a Playstation 2 is the optimum platform for gun games.
If you're sticking to emulators, it depends on how picky you are about how they function.
IMO, the Aim-Traks are the best choice for LCD screens, but the aim does not stay perfect when body position or height of person playing is changed, so you need to rely on the on-screen crosshairs instead of the gun sights. It's similar to playing gun games on a Wii.
There are a few other options for use with LCD screens, but they aren't as plug and play and won't work any better.
My personal preference is for the true light guns that only work with CRT TVs or monitors. This is what the guns were like when I was growing up.
I use the gun sights and the aim is accurate to within 3 pixels. The downside is that you can't use them on games that originally had mounted guns and spray bullets, like Terminator 2. The Aim-Traks will work fine with those games. The Act-Labs guns turn up occasionally, but you might have to keep an eye out for a couple months before you come across a pair. Don't buy the oldest version that connects via gameport instead of USB. They work differently and aren't compatible with as many emulators.