  We have proposed a program for determining the reference for the quasi-local
energy defined in the covariant Hamiltonian formalism. Our program has been
tested by applying it to the spherically symmetric spacetimes. With respect to
different observers we found that the quasi-local energy can be positive, zero,
or even negative. The observer measuring the maximum energy was identified; the
associated energy is positive for both the Schwarzschild and the
Friedmann-Lema\^{i}tre-Robertson-Walker spacetimes.
