Crusader Kings III: Royal Edition bundles the base game with its first Expansion Pass, which in turns features the game's first major expansion (Royal Court) and two flavor packs (Northern Lords and Fate of Iberia).

This is the edition you want to purchase if you want to eventually get all the CK3 DLCs and save yourself some money, as it costs much less than purchasing things separetely.

At the time of writing (April 21st 2023) the only DLC you won't get by purchasing this edition is the minor Friends & Foes event pack. The upcoming Chapter II Bundle, starting with the Tours & Tournaments DLC, will release next month.

While Crusader Kings III as a whole is great, my rating here is impacted by the quality of the DLCs included in this package.

Northern Lords and Fate of Iberia are solid pieces of regional contents, adding much needed flavor to regions that would otherwise play the same as others. Where they're lacking is mainly in historicity: compared to similar CK2 DLCs there are pretty much no historical events that can trigger (this is particularly glaring and disappointing with figures such as El Cid, that exist in the game but do nothing), and the starting setups (characters, realms, laws etc) could be much better. Northern Lords also doesn't do enough mechanically to distinguish Scandinavia from other tribal regions, and Fate of Iberia, despite the interesting new Struggle mechanic, suffers heavily from a lack of polish.

Royal Court, the first major expansion, is generally regarded as a disappointing entry given how long it took to come out and its cost. The Court related mechanics are often repetitive and uninteresting (if not downright annoying because of poor event weighting and bug fixing), and the Artifact system, while fun to play with at first, is marred by an awful UI and tends to trivialize other aspects of the game. The best aspect of this DLC is the revamped Culture system, which is partially included in the base game, and that, well, has little relation to court life.

As a whole all these DLCs do a good job at improving the vanilla experience, particularly in featured regions, but the various issues I mentioned mean they're too expensive for what they bring. The Royal Edition solves this with its lower price, and WinGameStore adds to that with their usual competitive offering.

The Horse Lords Content Pack for Crusader Kings II is one of the few extra DLCs you can purchase on top of the various expansions. More specifically, it doesn't come with the related Horse Lords expansion.

This DLC nets you a few culture-specific (Mongol, Frankish, Germanic and Italian) 3D models for the military units on the map that represent armies, plus custom portraits for characters of Cuman cultures, and finally Mongol / Steppe themed music that plays when you're playing a ruler of those cultures.

The 3D units are from my experience barely noticeable, whereas the portraits and extra music do an okay job in enriching the experience when playing in the Steppe.

I probably wouldn't recommend this DLC on its own given the price (you need to purchase 2 more Content Packs, namely Conclave and The Reaper's Due, to replace all of the ugly vanilla portraits), but WinGameStore constantly has great prices for Paradox DLCs, which makes completing your CK2 DLC collection much more affordable.