Shiva EX is the perfect fight in my opinion,

Always something different happening,
Moves that keep the tank awake,
The right amount of movement,
No cheap instant death mechanics, (mechanics where if you mess up you only take at most one guy with you, AND NOT THE WHOLE TEAM unlike coil. ),
Pre fight tactics (two people in the same safe spots),
Laughing at newbies sliding on the ice,
Recoverable,
A decent dps check,
Can be single or double tanked,

And best of all......

The boss is always facing away from the group (SORRY TANKS)