iPhone storage full, what's the fastest thing to clear?

Settings > General > iPhone Storage shows you what's actually eating it, go by that list rather than guessing. Usually it's video. Delete the big ones, then go into Photos > Albums > Recently Deleted and empty it, because otherwise the space doesn't come back for 30 days. That catches a lot of people out, they delete stuff and nothing changes. After that, Messages is often huge from years of attachments, and there's an offload option for apps you never open that keeps your data.