What's your go-to email popup plugin for WP?

For blog.

Looks like you haven't used https://sumo.com/ yet. Its the best and we are using it at http://smartvectorpics.com/

Its pretty good.
Zero cost
You can build email list
You can direct people to your Social Media accounts
and some other features

For an ecommerce site or otherwise?

I hate popups but unfortunately, for ecommerce they do tend to work.

I've been pretty happy with sumome and their set of (freemium) plugins.