I’d argue for Seleucus Nicator, after the death of Alexander he successfully seized power in the majority of the late and the great’s empire and indeed was able to march all the way to India once again, winning a large number of battles along the way, but ultimately lost the war with the Mauryans and moved on. This was around 300 BC and he died around 280 BC (give or take, I’m just doing this off my head). His son, Antiochus Soter, was able to avoid what had happened to Alexander and kept his father’s empire successfully United, even able to expand its power somewhat. His successor, Antiochus II was nothing terribly special but was able to keep his empire somewhat unified. His successor, Seleucus II was much the same but the Seleucid Empire. Each of them ruled for a further twenty years apiece.
His successor, Seleucus III, died after two years so I can’t say much about him. But his successor, Antiochus III “The Great” was something to be admired. He ruled for nearly 40 years and successfully expanded quite a bit. His only real mistake was going up against the Romans, who were able to subdue him and destroy the Empire for the most part. When he died, his successors continued to rule Syria for a hundred years.
While the actual empire building fizzled out after Antiochus I, the successors of Seleucus were able to maintain a fairly powerful state for 120-ish years until Antiochus VII died and left the empire confined to Syria.
The empire did fall quickly in the grand scheme of things, but there’s plenty of potential for a timeline where they stick around for a thousand years if it’s played right.