Email Lead Genration

What is effective way for double opt in lead generation ?

Legit leads? Offer them a reason to click that confirm button in the confirmation email, ie; give away report, video, something of value.

The only way is the Inbound Marketing way where you first create valuable content for your target audience. Let them find your site on google and visit. Lure them to a landing page using calls to action. There you should have a "gated offer". In order to access the offer (an ebook or a more valuable content) they'll most probably turn themselves in to get converted into leads.
You can achieve this by free Wordpress plug-ins and a free Mailchimp account.
Google 'inbound marketing' and start learning. Hubspot offers free courses..