I think that my favorite is Halo 3, because of the great campaign and the great multiplayer.
They are a all the best in their own special way… But my favorite is H2/H2A.
I don’t know if it is the best but Halo 3 ODST gets my vote.
H3 takes the cake.
This isn’t really a topic for the Halo forums, we have plenty of topics like this in the Polls section and in GD 