Designated by the North Atlantic Treaty Organization as “Flanker-E” the Soviet-derived Su-35 fighter jet became the definitive production variant of the Su-27 in the early 1980’s.
Though the Chinese J-20 and Russian Su-57 aircraft are likely highly-capable warbirds, some U.S. aviation experts have suggested they shouldn't rightfully be considered "fifth-generation."
Reports from Russian state media have suggested that Sukhoi was working on three variants of the Su-75: a single-seater, a two-seater, and an unmanned version.
The biggest knock against the Su-57 remains its numbers – as Moscow simply doesn't have that many, and won't likely ever have the Felon in significant numbers.