The Lord of the Rings Online™ did well with its endgame under Shadows of Angmar™ but has struggled since. The SoA endgame had two sets of three full-group instances, of length varying from three large rooms to three hours. Late in SoA, there were full-group battle instances that were precursors to skirmishes. Small fellowship instances had yet to be developed. The two raids were big deals, Helegrod less popular because it took more effort to organize 24 people, but there are still regular groups for The Rift.
The endgame in Mines of Moria™ was deeply marred. The six instances were known for bugs and exploits, and the current player approach seems to be farming the fastest and avoiding the two that are painfully long and/or difficult. The developers have judged Radiance to be a failed mechanic (as implemented) and are removing it. MoM launched with a one-fight raid, later added another one-fight raid, and at the end of its life added something comparable to The Rift. I have no idea if that ever became popular; the small fellowship instances never did, but the full-group Halls of Crafting still sees regular use. The small fellowship instances faced the problem of using mirrors, levers, and gates to create puzzle content. Group puzzle content does not work well aside from the occasional puzzle boss.
Siege of Mirkwood™ is about to get its second shot at an endgame, and it needs it. It launched with three small fellowship instances, a full group instance, and a two-group raid. Aside from skirmishes and scaling content, that has not changed in over a year. Of these, the ones seeing the most use seem to be the fastest small fellowship instance and the full group instance. (LotRO has consistently had some fun full-group instances. SG is a good dungeon.) I enjoy the Warg Pens, even if the challenge mode there is a bit tedious. The Dungeons of Dol Guldur is just poor on every level; it is a neat idea, but stealth content + groups + MMO does not work well together, to say nothing of its boss fight (he can repeatedly disable the entire group with no way to prevent it, and if he randomly does one move back-to-back, he kills half of the prisoners you are rescuing; combine the two for bonus fun) or its deed that requires running it many times (save 10 named prisoners, a random 2 of which count per day, and of course you must successfully get those two out). I am not much of a raider, so I cannot comment on whether people like BG or are just dutifully running it because it, like DN, was the only raid content out there. Oh, and it can give the best loot, so of course people are running it for the reward. I hear about loot, but I have yet to hear anyone argue for it on the basis of being fun. And you only need that loot if you are running the BG raid, so it is neatly circular.
If GLFF is any guide, the most popular and widely used SoM endgame content is the level-scaling SoA content, the Great Barrows coming in #1. Which is kind of sad, if the first dungeon in the game is the most popular dungeon at the end of the game. Let’s hope things go well in the next update.