the microcanonical ensemble
This is the ensemble for a perfectly isolated system: fixed energy, fixed volume, fixed particle number, exchanging nothing whatever with the outside world. Picture a gas sealed inside a rigid, perfectly insulated, closed box. Every microstate with the allowed energy stands on completely equal footing with every other.
The constants of the macrostate are E, V and N. By the fundamental postulate, each of the W(E, V, N) accessible microstates, all with energy in a thin shell from E to E + dE, carries equal probability 1/W. The thermodynamic bridge is Boltzmann's entropy S(E, V, N) = k_B ln W, from which temperature, pressure and chemical potential follow: 1/T = (dS/dE)_{V,N}, P/T = (dS/dV)_{E,N}, and -mu/T = (dS/dN)_{E,V}.
Conceptually the microcanonical ensemble is the foundation; every other ensemble is derived from it by allowing exchange with a reservoir. In practice it is the hardest to compute with, because counting states at exactly fixed energy is awkward, so one usually switches to the canonical ensemble. In the thermodynamic limit all the ensembles give the same averages, an equivalence that holds even though their fluctuations differ.
N ideal-gas molecules sealed in a rigid insulated box of volume V with fixed total energy E: counting the phase-space volume on the energy shell yields the Sackur-Tetrode entropy.
Fixed E, V, N is the setting in which S = k_B ln W is most directly at home.
The energy is fixed only within a thin shell dE, not to a mathematical point; the entropy is insensitive to dE because ln of the shell volume is dominated by its huge exponent, not its thin width.