I like to find blades that compliment one another.
I'll call it the "Rock Paper Scissor" approach.
The main idea is to be prepared for any type of cutting task, and therefore to carry (usually 3) knives that excel at different tasks. Each knife should be able to do something noticeably better than the other.
With a system like this I am never wishing I had a thinner blade or a thicker blade, a blade with more belly or a straighter blade, a smaller blade or a bigger blade, and so on, etc. etc.
I implement this system with a keychain sized knife for starters. It is a knife that I never forget to carry 24/7-365. Second, I usually carry a pocket knife -of the type that actually sit in a pocket. This will usually be a traditional slipjoint with multiple blades. Sometimes it is a Leatherman or SAK as well. In any case, the second knife that I carry is usually the most useful since it will have multiple blades and/or tools. Often times, the second knife will be enough to stop me from carrying a third. When my second knife leaves more to be desired, I'll carry a third knife. -This will generally be a modern folder and its design features will depend on what my second knife is. The third knife will usually have a pocket clip and a one-hand opening design.
I carry the entire system in both front pockets, and sometimes use my back pocket for the modern knife.
I must note though, nothing is set in stone.