The WordPress plugin WP Rocket is one of the most popular cache plugins. It combines caching with features like file optimization, lazy loads of images, preload and priming of cache, cdn connection and more. It is very reliable and easy to use.
According to our tests a good, and free, alternative to WP Rocket could be a combination of the plugins WP Super Cache and Autoptimize. Another good combination is Cache Enabler, Autoptimize and webp image files created by ShortPixel.