Which substance has the highest temperature range between melting and boiling point
Gallium melts at 30 &deg;C but doesn't boil until 2200 &deg;C.  If 30 &deg;C is a bit too warm to count as "room temperature" or "normally" for you, I found an old paper that recommends tetralkyl silanes such as tetradodecyl silane as lubricants that are liquid over very wide temperatures.

Addendum:

Dowtherm A is a eutectic mixture of biphenyl and diphenyl ether.  According to its manufacturer:


  These compounds have practically
  the same vapor pressures, so the
  mixture can be handled as if it were
  a single compound. DOWTHERM A
  fluid may be used in systems employing
  either liquid phase or vapor phase
  heating. Its normal application range
  is 60°F to 750°F (15°C to 400°C),
  and its pressure range is from
  atmospheric to 152.5 psig (10.6 bar).


[...]


  The viscosity of DOWTHERM A
  fluid is low and changes only
  slightly between the melting point
  of the product and its top operating
  temperature.