The least efficient was the recruitment job fair. If you consider the costs of recruiting through each channel, then employee referral and college recruiting look pretty good. These metrics will vary by company. But it's important to keep and use them to refine your process. Look at it this way.
Most companies have elaborate forecasting and tracking systems for keeping the sales pipelines full.
They know lead times, visits or calls per sale and costs per sale. They segment their customers in different demographic and statistical groupings, and they pay for leads. They call lapsed customers and try to win them back. If you're a manager looking to hire IT professionals, your first action item is to find out if your HR people keep such statistics. If not, push them to do it, or do it yourself.