25th-order high-temperature series are computed for a general nearest-neighbor three-dimensional Ising model with arbitrary potential on the simple cubic lattice. In particular, we consider three improved potentials characterized by suppressed leading scaling corrections. Critical exponents are extracted from high-temperature series specialized to improved potentials, obtaining γ = 1.2373(2), ν = 0.63012(16), α = 0.1096(5), η = 0.03639(15), β = 0.32653(10), δ = 4.7893(8). Moreover, biased analyses of the 25th-order series of the standard Ising model provide the estimate ∆ = 0.52(3) for the exponent associated with the leading scaling corrections.By the same technique, we study the small-magnetization expansion of the Helmholtz free energy. The results are then applied to the construction of parametric representations of the critical equation of state, using a systematic approach based on a global stationarity condition. Accurate estimates of several universal amplitude ratios are also presented.