I picked out the four record attachments as the best designed cards for the mono-sphere archetype the designers were trying to push during the Against the Shadow cycle. What's so well designed about them, you may ask? Well, for one thing they're not absolutely exclusive - you can play them outside of mono-sphere, but for a higher cost. This makes sense for the setup, because it means that the powerful effect is generally available, but is more easily accessible to someone who chooses to focus on one sphere only rather than blending them together. Furthermore, it provides mono-sphere in particular with access to an effect which is likely more useful for them than it is for dual or tri-sphere setups, because a mono-sphere deck is likely to have a wider variety of events it may wish to recur in its chosen sphere, while a multi-sphere deck will have a mixture from multiple spheres. Which leads us onto the design of the cards themselves - they give tremendous flexibility since they allow you to choose any event in your discard pile at the moment you choose to discard the record - so long as it's an action not a response - in contrast to other recursion effects which would have you choose a specific card to return to hand or shuffle back into your deck. Moving the event subsequently to the bottom of your deck neatly limits the recursion (so multiple copies of the record or attachment recursion won't allow you to immediately recur the same copy of an event multiple times), but in turn allows for additional recursion if you have enough draw or fetch effects to retrieve the event from the bottom of your deck. And then there's just the general uses of recursion - it works for playing events multiple times, but it also works very well for any strategy which involves discarding a lot of cards (such as Noldor or Caldara) since with a record in play it's like every (non-response) event in your discard pile is in your hand. Between the flexibility, the power of recursion and enabling some different playstyles for mono-sphere in terms of your attitude to your discard pile, I'd say it's hard to argue the records aren't incredibly well designed. All that remains is to examine the sphere-specific aspects of each record.
So the Map of Earnil in . At first glance this seems less useful, since the events you most want to recycle are responses - well, mostly just A Test of Will, but sometimes getting multiple Hasty Strokes could be good too. The Map doesn't do those however. On the other hand, if recycling your cancellation is super important, you can use the Map to recur Dwarven Tomb to recur either of the cancellation events and play the same copy 3 times for one event, one Tomb and one Map (4 times if you manage to retrieve the Tomb from the bottom of your deck afterwards). But those two are far from being the only important events. Being able to recur threat reduction like Elrond's Counsel or The Galadhrim's Greeting can have a huge impact over the course of a full game, especially if the quest is threat-heavy. In a Caldara deck of course being able to play Fortune or Fate as often as possible is very important. Lay of Nimrodel for big quest pushes, Shadows Give Way for ultimate security in combat for multiplayer games, Stand and Fight to muster more allies, or perhaps Power of Orthanc for quests with nasty conditions in them all strike me as good options as well - and I will once again remind you that you don't need to choose which event you recycle until you actually do it. The final two points I would make are that mono- is perhaps a more common setup than some other mono-sphere options, since with the release of more combat-capable and neutral characters it has become more able to do everything, and it also can work simply if you just want a very focused, dedicated questing deck. The second point is that the sphere also has the best natural synergy with this kind of discard shenanigans - between Caldara, Stand and Fight, Dwarven Tomb and Will of the West, they can already bring things back from their discard pretty well, and they also include multiple effects to discard those cards in the first place such as Éowyn, Arwen Undómiel, Círdan the Shipwright and Zigil Miner. That natural synergy sets up to work incredibly well with its Record attachment even if one might argue that it doesn't have quite the juiciest targets for it.