Best Zone: Westfall and Darkshire were great while levelling, they had the most epic feel to them, both the plague lands were fantastic at L60 (it was also super cool to see random vanilla quest giver Tirion Fordring play a major role in WotLK). Also Hellfire peninsula, simply because of how epic it was to step through the portal.
Least Zone: Desolace soooooo dull/boring, perfect name though.
Best Raid: Naxxramas, best designed/layed out raid ever and the amount off bosses, variety and fight design is still unparalleled.
Least Raid: The cho'gal one, most forgettable of all raids.
Best Boss: Any Naxx one really, Horsemen/Thaddius especially.
Worst Boss: Prince in Kara, didn't matter how well you played or how well geared you were, if the RNG didn't go your way you were toast, terrible design.
Least Zone: Desolace soooooo dull/boring, perfect name though.
Best Raid: Naxxramas, best designed/layed out raid ever and the amount off bosses, variety and fight design is still unparalleled.
Least Raid: The cho'gal one, most forgettable of all raids.
Best Boss: Any Naxx one really, Horsemen/Thaddius especially.
Worst Boss: Prince in Kara, didn't matter how well you played or how well geared you were, if the RNG didn't go your way you were toast, terrible design.