Battle of Manzikert
Advise Romanos to not trust Doukas and make sure the right wing understands the retreat order before the battle. Would have resulted in the withdrawal to a more defensive line, making sure it didn't end in a decisive Seljuk defeat and the drawing out would've probably led to the romans winning the war after a long time. Byzantine Control over Anatolia would remain strong and the events setting in motion the ciivl war and decline of Byzantium would not have happened. Neither would Anatolia be turkified and Byzantium with the Turkish invasion of Seljuk could have regained large parts of it's former land, exploiting the Seljuk weakness instead of vice versa and would've probably stayed a regional power, controlling Greece, Anatolia and possibly reconquering Levant. Very noticeable as it could've survived then and not collapsed (possibly at least)