Unfortunately, there is no best way. It all varies by your goals, the funnel required to get people to the conversion and the value of each attribution. This type of modeling takes time to establish and generally will only come through experience of analyzing the data of visitors going through the website.

Great question and I would go to the Google Analytics Guru Avinash blog post on the subject since it hasn't changed in the past 3 years really.

http://www.kaushik.net/avinash/multi-channel-attribution-modeling-good-bad-ugly-models/

If you are familiar with Google Analytics, the best place to start is with the basic attribution reporting section. Look at which channels are better at assisting conversions and which tend to be better at closing conversions.

Then, tie it up with whatever your current marketing goals are.

So if for example you see that social media is assisting a high number of conversions, but you've spent no marketing resource on it to date, you might want to review your social media activity to capitalise on this important touchpoint in your customer's conversion journey.