The most important factors have to be incrementality combined with scalabilty. Naturally the shorter the attribution window will be then the more accurate the representation of what this channel is actually doing for your growth will be. Unfortunately, by doing so, you're going to lower the RPMs associated to your campaign which means that this channel will deprioritize your campaigns in their waterfall.
Multiplying the incrementality percentage by the number of users you're getting at various attribution windows will give you a true representation of the growth you're getting. Whichever score is the highest would be the best fit for this particular channel.
Every time you add a new channel you should test out how this affects the incrementality of your previous channels to see if this impacts your collective score of all of these channels combined.
This becomes even more effective if you can link the user growth to projected revenue value.