Trying NOT to be a BC fanboy here but BC had the best heroics. They were diffucult but very rewarding. I liked the amount of rep gained from doing specific dungeons for specific factions. It just made sense to earn Cenarion Rep for doing the Coilfang dungeons.
My favorite of those groups was Steamvaults. I ran that at least 200 times as a druid tank and it was so much fun.