There are many channels for lead generation. The list includes:
Social media marketing
Mobile apps (ASO)
VSO (Voice Search Optimization)
But i think e-mail marketing still rules the roost. it is because users don't have to spend hours on searching about your product in their news feed, SERPs etc. They just get all the information delivered about it straight to their inbox.
Therefore, if users are interested, they click-open the e-mail and land on your webpage for purchasing the product, You will be amazed to know, e-mail marketing converts 62% visitors into leads. This fact makes e-mail marketing the king of lead generation. More importantly, all of the leads are organic.
Then comes the number of SEO (Search Engine Marketing), ASO (App Store Optimization) and VSO (Voice Search Optimization), PPC, Blogging and SMM (Social Media Marketing).
Lead generation through SEO is very much dependent on paid ads, SMM, organic techniques. But the lead generation rate of SEO is very less as compared to the e-mail marketing.
ASO the counterpart of SEO. It is the form of SEO used to get mobile apps ranking high and installed in Play Stores. All app downloads ensure organic and new lead generation opportunities.
Now talking about VSO, It is a speech recognition technology. You have get your website and app optimized for it to increase the chances of your website's and app's search engine appearance.
Both ASO and VSO fall in the category of Strategic Digital Marketing.
But none of these ways match e-mail marketing in-terms of lead generation.