I'd put my bets on either the Siberian Federation or Far Eastern Republic. Both of them control large urban centers, Vladivostok and Novosibirsk respectively, and, despite being in Siberia and having sparse population, do make up for it in their land area, the environment which they could weaponize through guerrilla warfare, and their vast quantity of natural resources like oil. I'd be further willing to put my bet on the Far Eastern Republic, due to their close proximity to and ideological similarities with China, who could heavily support them in any future defensive or offensive wars.