Centre
The centre channel is the most important speaker in any Home Theater system. It carries almost all of the dialogue, and much of the other on-screen information. At times the centre speaker is responsible for over 70% of the total soundtrack.
It is also essential that the centre speaker sonically matches the left and right speakers. It is common for the soundtrack to move from one side of the room to the other passing through the centre speaker on the way. If the centre speaker is sonically different to the left and right you will get a change in tonal character as the sound passes through it.