Chemistry 3

http://www.cem.msu.edu/~cem251/F96/alkane_conformations/cem3.html

$0^\circ, 360^\circ:$ Full eclipsed with none;

$180^\circ:$ Anti with -5.0;

$60^\circ, 300^\circ:$ Gauche with -4.1;

$120^\circ, 240^\circ:$ Eclipsed with -1.5;

Where -5.0 is not unstable, none would be least stable.

Answer: $\boxed{Full~eclipsed}$