I'll preface this by saying that I'm not opposed to carrying people through content they otherwise wouldn't be able to complete in the process of pursuing my own goals. Hundreds of runs, both random and premade, involved brute forcing other members of the group through to the end for the chance of getting what I'm looking for.

That said, it's irritating, infuriating, even, when the benefit for all of that effort is directed to the party member with the least contribution to the success. So many times while farming style pages, leads, motifs, and other rare drops, there will be those people clueless as to what is happening, then walk out of the dungeon with something you may be on your 20th attempt at receiving. It's one thing to be magnanimous, but doing multiple runs for the item you desire and watching it go to someone else without so much as a thank you or acknowledgement of your existence begins to feel like you're just being walked over and wears on your tolerance for anything less than perfect in future attempts.

Groups at a basic level work toward a common goal and interact with the same items as everyone else, and the death of the final boss is the end of their obligation to one another. Why, then, is the quality of the reward not shared within the party? Everybody gets gold, a trash item, a monster helmet, and a purple set item that everyone can trade amongst themselves with equal value, and then out of nowhere this one person gets a golden style page. It's a major disincentive to want to continue running with subpar players.

As the title says, the rarest loot should be dropped for everyone that ran the dungeon. Similarly to how an incentive for doing a vet hardmode is a guaranteed motif page for everyone involved, leads and style pages being group-wide give players a reason to struggle through the content together. Even in premades, unless I'm running with the closest of friends, the majority of my experiences aren't "I got what we were looking for, but I'll stick around to help you get one, too," it's nearly always something like "lol I got it kthx bye", resulting in the added effort of finding a new member to replace the lost one, hopefully at the same or better quality, and time lost waiting to fill the slot.

All in all, the unreliability of strangers in a premade and the possibility of your desired reward being given to the weakest link lead me to be of the opinion that all members involved in a successful run should all be given the rarest loot if it happens to drop for anyone.