Best is slightly subjective as it really depends on your needs specifically :-). One of the best things you can implement is caching, in which case I’d recommend WP Super Cache and W3 Total Cache.
WP Super Cache is easier to setup and requires far less configuration. W3 Total Cache, on the other hand, is far more complex and offers way more options. If you prefer to be hands on, and want total control over caching and need options when it comes to how caching is handled (Redis, OpCode, Memcached, File Cache, etc), W3 Total Cache would be the plugin for you :-).
W3 Total Cache also offers easier CDN integration, so if you’re offloading assets (images, CSS, JS, fonts, etc) to a CDN that’s supported, that’s another benefit.
I also generally recommend iThemes Security (Better WP Security), as it can help on the other end of the spectrum by providing a pretty complete security suite to deter would be attackers.