  Ramsey interferometry allows the estimation of the phase $\phi$ of rotation
of the pseudospin vector of an ensemble of two-state quantum systems. For
$\phi$ small, the noise-to-signal ratio scales as the spin-squeezing parameter
$\xi$, with $\xi<1$ possible for an entangled ensemble. However states with
minimum $\xi$ are not optimal for single-shot measurements of an arbitrary
phase. We define a phase-squeezing parameter, $\zeta$, which is an appropriate
figure-of-merit for this case. We show that (unlike the states that minimize
$\xi$), the states that minimize $\zeta$ can be created by evolving an
unentangled state (coherent spin state) by the well-known 2-axis
counter-twisting Hamiltonian. We analyse these and other states (for example
the maximally entangled state, analogous to the optical "NOON" state $|\psi> =
(|N,0>+|0,N>)/\sqrt{2}$) using several different properties, including $\xi$,
$\zeta$, the coefficients in the pseudo angular momentum basis (in the three
primary directions) and the angular Wigner function $W(\theta,\phi)$. Finally
we discuss the experimental options for creating phase squeezed states and
doing single-shot phase estimation.
