Easy, Yoast SEO. It is filled with great features and it's free! You can edit .htaccess and robots.txt, optimize RSS feeds, create multiple XML sitemaps on the fly, in addition to adding title and description tags.
It is the one plugin that goes in everyone of my WP installs.
I'm using WordPress SEO by Yoast from last 2 years and find it very helpful for both on page and off page SEO. It is easy to configure and use. You can also easily integrate Google, Bing, Alexa, Yandex and Pinterest verification with the help of this plugin.