Good question...!

It's quite difficult to determine what the most popular electronic resource is, as 'popularity' can be subjective. A resource could be popular in that it sees the most downloads overall, but it could be popular because it's the most suitable resource for a particular subject area. 

If you're interested in how electronic resources are used as a whole, and what their usage can tell us, take a look at LISTA (Library, Information Science and Technology Abstracts) which you can access from the A to Z List of Databases, and if you want the University of Bolton take on resource usage versus popularity, you might find this blog post interesting.