This is such a important question which almost got a very default answer as "all the organs are important" If you insist a person to answer then its either heart or brain.
So whats the important thing - I guess the brain is very much complex as it controls all the voluntary and involuntary organs -- in future may be we might imitate the heart (with artificial heart pumps and all) but for brain it would be very complex to imitate (yes - we have got robots based on artificial intelligence - but they are no where near humans) Again all the signals are received by brain as it makes the decision accordingly.
Offcourse eye and ears are very important as they are sensitive and you have to enjoy the visual and sound effect.
Brain is anyway the most important - as its your identity - its thinks - it makes the decision - it enjoys and it controls / singals the body