Not come across a decent raid since LOTRO's Rift (part of the old Shadows of Angmar game). There were no "hard modes", no easy versions. Just a very well put together multi-boss raid that enticed you in with a couple of straight forward boss encounters and that got progressively ever more difficult. Even after the the Moria and SOM releases and two level cap increase, the final boss (Thaurlach) was still very hard.
Ya that was good stuff. I was with one of (possibly the? Didn't check world ranks at that time) first guilds to have cleared it, Maelstrom. Spent months hitting that raid.
But honestly..after awhile it was just going through the motions. I think Ost Dunhoth may have been the biggest challenge in raiding I saw in that game (though you could actually switch to normal mode to clear locks and get to the end bosses on hard - someone claimed world first final boss kill by doing this, which most considered cheesy).
I think it took most top end guilds a couple months to get the entirety of Ost Dunhoth cleared, and that's the way raiding should be I think. People say 'lotro was ez mode raiding' but..some encounters such as these were actually very challenging.