Go tell the Spartans,
Passerby,
That here, obedient to their laws,
We lie.
- Simonides (Epitaph for the Spartans who fell at Thermopylae)
I thought ODST was a great departure from the norm, gave a bit of perspective to the whole series. It wasn't as challenging as the others, however. But still probably my favorite, next to Reach.