My most often used knife is always on my keychain, the frequently praised Fallkniven U2. It's not too big or bulky but has a big enough blade to handle most things I need to cut throughout the day. With it's blade geometry and amazing edge holding ability it remains my favorite, even though it is cheaper than most of my knives. Another great keychain knife is the Spyderco Dragonfly, which is also a great slicer with excellent ergonomics.