Be sure to check the laws in your state. Many have restrictions on blade length and or types of opening mechanisms. That said, I've found that the Gerber "assisted" models open in a heartbeat and are of good quality. You can usually find them discounted on THEbay etc. I tend to carry the smaller (onion) series as a money clip but them make a variaty of styles and sizes.
Assisted Opening Knives | Gerber Gear
p.s. Be sure not to bring a knife to a gun fight!