I'd say my favourite instance would have to be the deadmines. I know it's for noobs but it was my first instance and I had a really good team that eventually became my guild.

Arms/Fury Warrior (Tank/DPS)
Holy Priest (Healer)
Marksman Hunter (Puller/DPS)
Fire Mage (Ranged DPS)
Feral Druid (Substitute Tank and Healer)

My least favourite is Scarlett Monestary