As with all “what is the best” questions … I have to issue my little disclaimer. There is no, single, “best” anything for everyone. It all depends on what YOU need, your budget, and what simply feels “right” to you.
Beyond that, my favorite email subscription plugin for WordPress is OptinMonster.
It allows you to build different types of opt-in forms in minutes. Everything from timed pop-ups, to exit intent pop-ups, to mobile opt-in forms, to click based opt-ins, to full page opt-ins, to floating bars, and more!
OptinMonster integrates with nearly every autoresponder service out there. They have split-testing or A/B testing capabilities built in. Integration with Google Analytics.
All you need to do is pick your design, customize it to your needs, connect your autoresponder service, then publish it to your site.
Oh, and it's affordable – at $199/yr with the Pro Plan.
So, at this point in time OptinMonster is definitely my favorite subscription plugin for WordPress.
Another great plugin that I used to use a lot; however, have pretty much replaced it with OptinMonster is OptimizePress … and it's a bit different than OptinMonster in that it really focuses on Landing Pages as opposed to just the opt-in form.
So, if you're looking to create great looking landing pages, I'd recommend checking out OptimizePress. It does have a few different prices ranging from $97 to $297, but compared to a landing page service like LeadPages or unbounce, it could save you money in the long run!