I run my own event-focused SaaS agency built upon WordPress Multisite, and I guide WordPress agency owners through the transformation framework via the Trailblazer FM Podcast and our in-person events.
1. Bricks Builder
This is a theme which provides an awesome page builder with more features than you can shake a stick at. Importantly it removes most of the requirements for other third party plugins depending on the site use case. Sites built in the Bricks Builder are normally super fast also beating page scores on Google Insights.
2. The SEO Framework
The SEO Framework gives us all the basics we need, without the bloat and is SUPER easy for clients to use. No subscriptions needed (although some premium options available if you have some extended needs).
3. Antispam Bee
No bloat, no ads, open source tool that stops comment spam for us and clients. Antispam Bee is super easy to use, with no subscriptions etc.
4. Query Monitor
We activate Query Monitor when we need to work out if a plugin, or some of our code is causing problems. Helps us highlight issues and resolve so we can keep our sites performing at their best.
5. Gravity Forms
Gravity Forms is the best form builder for use with multisite. Has specific settings to allow us to control how it is used across all networks. Is a mature product, great support, and documentation.