These are the strongest internal matches for this guide topic right now.
Glastonbury sits at the top by capacity, followed by large-scale events like Reading and Leeds, Download and Creamfields. They are not interchangeable though, so size alone is a bad shortcut.
Start with genre, then decide whether you want a huge commercial weekender, a curated boutique event or a one-day city festival. That gets you to the right shortlist much faster than browsing by reputation.