WordPress by itself is a powerful content management system, but not at all optimized for everything, such as spam prevention or system optimized. Here are 5 must-have WordPress plugins. I know, it may sound a lot, but hey, these are the plugins that gets your WordPress going in long run!
Jetpack

Packed with some advance features (the good thing is you can turn on or off and features you deem unwanted) such as social sharing, image optimization, mobile version and etc. This is definitely a must have if you are playing safe with WordPress theme.
BackWPup

Seriously, why can’t WordPress has a built in back-up system, so we all can have a rest? Well, this is one of the plugin that give you a lot way to back. To the server folder, to your Dropbox, or even, to your email (We love this!)
WordPress SEO

Not all theme are built with SEO in mind, and definitely not those free theme or update-one-a-year theme. WordPress SEO here it rescue! This plugin not only help you with your SEO, it suggest you on your keyword and how-to.
WP Super Cache

Again, why isn’t this built-in with WordPress? Every websites need a cache system, be it WordPress or any other content management system. WP Super Cache is so simple to use, once you setup, you’ll probably forgot about it.
WordFence

There is no software that is 100% bulletproof, but at least, some developers are helping to keep software secure. WordFence is one of it for WordPress.
Hope this give you an idea where to get started. Let us know if there is any great plugin we should check it out!