While Shadow T2C is certainly good because it required efficiency, that's really the bulk of its appeal. Sure, it could be done quickly--but who wants their fun over quickly? I certainly want the boring fights over quickly, though, that's for sure.
Since this is a video game, I prefer fights that are actually fun *and* challenging. Thrang. Thaurlach. Thorog. The Blind One. The Lieutenant. (anything that starts with "Th", apparently )
I think overall, it would have to be Thaurlach. Cool story, a great lead-up fight in Thrang, movement, positioning, splitting into smaller groups (the strategy involved in doing so was a ton of fun), actual rewards for completion, the ability to be successful even without 12 totally hardcore 100% efficient people...raiding didn't feel like work back then. It was more inclusive and social.
For many many reasons, they'll never top that fight. Though after all these years, whenever I start randomly thinking about LOTRO, I always mouth the word "Thrang". Maybe it's just a cool word. Or maybe we spent so long working out strategies for that fight (only time I've ever fought a boss with a pitchfork) that it's permanently imprinted on my brain.
[color=red]Work like no one is watching, dance like you don't need the money...[/color]