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 Tome of Atanatar in . Now I should mention that I don't actually play a whole lot of mono-, generally pairing the resource generation with another sphere. But when I do go mono-, I'm always struck by how ridiculously powerful it can be. The Tome has arguably the absolute best targets for event recursion in the game with Sneak Attack (to combo with Gandalf, naturally) and Strength of Arms, which makes mono- phenomenally powerful all by itself without any other help or the ability to recycle it. Honestly I think I could just stop there and consider that a reasonable case for marking the Tome down as the most powerful Record, but let's continue. Reinforcements can be great for similar reasons to Sneak Attack, and is even better in multiplayer. A Very Good Tale is another incredible card in the right deck, and being able to recycle it (including perhaps if one use of AVGT discarded another copy of it, which can otherwise be a hazard of using it) just mutliplies up the rate of your ally mustering. Lure of Moria for Dwarves and Grim Resolve for everyone if you have the resources (which in you quite possibly do) are incredibly powerful effects similarly to Strength of Arms. Speaking of having lots of resources, a Dwarf deck could get considerable value out of recurring We Are Not Idle, or if everyone's happy to eat a threat raise, Legacy of Númenor can boost everyone. Finally, in a 3 or 4 player game, just getting another play of Campfire Tales could be well worth it for the round-the-table benefit to everyone. While not quite on the level of , the sphere does have some ways to discard cards between Erestor, King Under the Mountain and A Very Good Tale, and it also likely has the resources to just play its events as it draws them, then recycle them afterwards. Plus there's always the option of Second Breakfast to bring back the Tome after using it as well. I would say has a majority of the massive game-winning effects, and being able to use them more often can just get ridiculous.