I made an actual record farm using a name tagged trapped skeleton at one end of an underground corridor and section in the middle i could,get creepers to drop in from above, then i go to the other end and the skeleton tries to shoot me whilst all the time pinging the creepers.
My creeper farm, The creepers land on hoppers so when they die their drops get funnelled to a chest next to where i stand.
An easier way is just to have an outside area with fenced off one space wide channels and an area behind you can be safe in. Get the creepers and at least one skeleton interested, and retreat to your enclosure and maneuver the creepers and skelly into one channels and get in position close enough so the creeper wont detonate but the skeleton tries to fire at you. Its quite easy with a bit of practice and understanding their path finding. Just need a large fenced area for yourself and a few one space channels about 5 fences long.
I have been playing for quite a while...have more diamonds and bling armor than a Louisiana rapper; but lo and behold, I only have 2 records ! Any suggestions, fellow crafters on getting these?
From the chest loot tables only "13" and "cat" can be found.
For the rest, a music farm (either a dedicated one or as an option attached to a more conventional gunpower farm) is probably fastest...
WRT the style of collection system mentioned by Mr_M_Derman, an iron golem can be used to draw the skeletons' fire instead of the player. [If the named skeletons shoot past the creepers through a five wide wall of water at the golems, the golems will be fully protected. Minecarts will make moving the skeletons and keeping them in position easier.]
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Answering the title question: the rarest chest loot remains the enchanted golden apple; however, a number of the "Enchanted diamond X" items would also count if one specified the enchantment.
Taking the question another way, emerald ore might also be a contender....
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WARNING: I have an extemely "grindy" playstyle; YMMV — if this doesn't seem fun to you, mine what you can from it & bin the rest.
I got these pretty easily by trapping and naming a skeleton and then trapping creepers over and over like Mr_N said. I did it a very basic way. I made two piston traps, which are four pistons facing up surrounding one block. That block has a pressure plate on it and there is Redstone underneath. When you step on the pressure plate the pistons go up trapping you. Put a 3x3 ceiling on it to prevent waking out and skeleton death by sun light.
The two are separated by one empty block. Put fence up around them to channel mobs into the trap easily. Then you can just have this simple setup in a big empty field with no light. I recommend killing everything that's not a creeper so that more creepers can spawn. Also build a tower for safety and a good view of the area when hunting. Box in the skeleton on three sides so that he can't shoot you while you're trapping creepers. Don't wear thorns.
On a side note, creeper only mob spawners are a great build for survival because gunpowder gets used up by elytra so much. Consider making on of those first and then you'll have an easy supply but that's way more work.