Answer:
The most important technique for project evaluation is the net present value (NPV) which compares the present value of discounted cash flows against the initial costs associated with the project. The other two most important techniques used are the payback period (either regular or discounted) and the internal rate of return (IRR).
Depending on the company's needs, sometimes one technique might be used instead of others. E.g. technological firms generally use the payback period because most of their projects have a very short life, 1 or 2 years. Other times, you might have to compare different projects and even if they are not mutually exclusive, no company can dispose of money freely. It only invests in certain projects that have a minimum required rate of return.
But the basic technique, the NPV, is the most relevant in a sense that no project with a negative NPV should be accepted.