For me, it would have to be Ulduar Freya+3, Firefighter, Yogg0, Agalon, Prenerf Steelbreaker. All those bosses were prime examples of progression each encounters progression could span several days < or weeks> per encounter. In contrast ICC was 11/12 less then 3 weeks after they were even available then you got to fight a brick wall boss.
Although RubySanc is being released soon. I wish Blizzard will surprise us with Gundrak as a raid. It would give some extra filler until Cata is released in the holiday season.