Hmm ok with a good grp every instanc is easy.
I prefer Underborg (same reasons as you)
Slave (you just have to kill half of the Mobs)
Bota. (But the group has to know the instance)
Don´t know but everyone is afraid of Bota. Last two runs i did it in 1 Hour and 1hour 30.
First run (warri,rouge,mage,lock,priest), perfect grp.
Second (warry, rouge, melee shami, shadow priest, pala), also very easy.
With daily: 7 Badges for about 1-2 hours.
But as i said. Most people (i know) are afraid of bota..."oh its so difficult..."
Mecha is easy too (exept 3rd boss) but i did it to often ^^