With 5 Years of Experience in WordPress Development. I have seen many changes. I started by customizing themes and installing plugins and slowly got into plugin and theme development. During this process and having worked with more than 200 clients. There are a few Must Have Plugins that I recommend to wordpress Developers.
1. Contact Form 7: Any website is incomplete with out a contact form. And the Contact form 7 is all you need to create a contact form, be it complex, be it a simple form you can all do it by using the contact form 7 plugin
2. All in One Seo: With SEO playing a Major role in any websites Success, you cannot afford to not to use the All in One SEO plugin. It is what it says All in One.
3. Google XML Sitemap: You have to have a xml sitemap on your website for search engines to index and google xml sitemap plugin is a very simple and easy to use plugin for the same.
4. WP Maintainance Mode: Being a developer we know that we have to have a plugin that can keep the users away while the site is being developed and wp Maitenance mode fulfills that objective for you.