I always liked Citroen‘s flat twin and flat four from the 2CV and GS respectively.
But the one that always fascinated me was their Type P1, an experimental engine from the 1960s. An 1800cc two stroke V4, with a supercharger driven by a separate 200cc four stroke engine in the same block. With the boost controlled independently of the main engine it was sufficiently torquey that it only needed one forward gear and a torque converter. It can’t have been quite as brilliant as it sounds, as it obviously never went into production.