I have written a couple dozen add-ons, but they are only for styling changes to the templates, or adding features that can be achieved through template edits (like an indicator for the thread starter). That way, I can upgrade the core XenForo at any time, and not have to worry about merging modified templates. And for conflicts, I can simply disable an add-on and the forum will function properly without it. In other words, template edits are "hands off" for me, now that I know how to create add-ons. (It's really not all that difficult to make simple add-ons!)
A handful I use:
Auto Complete Location (it forces users to choose a location vs. letting them type in nonsense), Amazon/eBay URL Parser, Password Tools, Search Improvements, ElasticSearch Essentials, Dragonbyte Security, Change Content Owner or Date, Approval Queue Plus, User Activity (by XenConcept), Ignore More (allows members to ignore threads and forums), XenCentral Feedback System, Font Awesome Manager, and Security & Passkeys. Developers I use both free and paid add-ons from are Xon/Atelier Aphelion, AndyB, DigitalPoint, OzzModz, and AddonsLab, among a few others.
We also have a handful of add-ons that are useful for moderators, like Report Improvements, Warning Improvements, etc., to make jobs easier for our staff.
We have quite a few add-ons since we have a large and very active community, and there are use cases for each add-on we use. Thankfully, other than one or two add-ons, none are deal breakers and, if they ever go away, nothing is really affected all that much.
