Skyscale
The long one. Five stages, twenty-four collections underneath them, and the game tells you the order.
The skyscale is the biggest mount chain in the game and the best documented, because the game documents it itself. Five collections form the spine, each one unlocking after the last — Newborn, Saving, Raising, Troublesome, then Riding Skyscales — and each names the smaller collections it wants finished first. Every step below is ArenaNet's wording; connect an account and the ones you have already done drop out.
Complete the skyscale mount collections to unlock the skyscale Mastery track.
The gate before any of it is the Living World Season 4 chapter Heart to Heart. Expect this to take days rather than an evening: several stages are time-gated and no amount of gold shortens them.