Unison Research Unico is my favorite.
For reference, here's a partial list of integrated amps I've owned: Classe CAP-151, Audio Refinement Complete, NAD C-352, Bel Canto EVO2i, PS Audio Trio C-100, Primare I30, Unison Research Unico, Cairn 4808 North Face, Musical Fidelity XT100, Rega Mira 3.