I thought it would be useful to add the number of badges each one yields, 1 per boss.
So you can choose the instances of the level you want with the most badges (although after compiling the data there's not much variety).

Anyway - just in case it's useful (no. of bosses in brackets)

Easy
- Utgarde Keep: (4)
- Drak'Tharon Keep (4)
- Gundrak (4)
- Caverns of Time: Culling of Stratholme (?) Cant remember how many I think at least 4

Normal
- Azjol-Nerub: Azjol-Nerub (3)
- The Violet Hold (3) (difficulty varies depending on random bosses)
- Utgarde Keep: Pinnacle (4)
- Nexus: The Nexus (4 - I thought there is a 5th named guy before the 1st bossbut cant remember if he drops an emblem)

Hard
- Ulduar: Halls of Lightning (Ionar) (4)
- Ulduar: Halls of Stone (Brann event) (4)
- Ahnn'kahet: The Old Kingdom (5)
- Nexus: The Oculus (4)