Another vote for the Ontario RAT-1.
It's a broad question, a lot depends on your environment and value system, and knives are as personal as it gets, but I personally wouldn't hesitate for a second. I have folders that cost hundreds, but I bought one of these early on for about $70, EDC'd it for two years and never felt a lack. It's a solid, sturdy and remarkably versatile and well-thought-out WORKING knife design that performs remarkably well for a lot of varied tasks from the mundane to the extreme, it's been improved since I got mine, and I personally think it's a ridiculous bargain at the current going price of less than $30. Only real negatives are that the black finish (paint) on the blade (if you buy a black blade) and the clip (in either case) wear quickly, and it's a zero-status knife with knife snobs. It ain't sexy. No exotic steels, titanium or currently-fashionable handle materials, but great blade and handle shapes, decent steel, full-length open stainless frame and a solid lockup. At $30 you won't be reluctant to really use it for anything and everything, from opening packages to spreading peanut butter, and after a few years of really using it you will KNOW that you can rely on it. I'd rather have the humble RAT-1 in my pocket when things get really interesting than a lot of $150-$200 folders that I could name and some that I've bought. At $30 a shot you can likely afford to have one not only in your pocket, but in each glove compartment, briefcase and pack- something most folks are not likely to do with expensive folders.