A bit of an old thread. However, I'm new so here's my take on the topic
1. Jetpack - So many helpful features, this really gets rid of a lot of unnecessary plugins.
2. iThemes Security - Better safe than sorry. You really need added security even if you think you don't need it. We've blocked a lot of brute force login attempts because of this.
3. WordPress SEO by Yoast - I need this for SEO (duh) & breadcrumbs
4. Titan Framework - My own plugin. We need this since the sites we deploy are built around Titan Framework. Titan makes it very easy to build customizer options.
5. W3 Total Cache - We only use some functionality like page caching. No minify since it can create unwanted results.
6. ImageInject - This isn't installed on every site, but if it's used for blogging then this really helps for those featured images. Kudos to @thoefter
7. CloudFlare - Not a plugin, but you know what it is.