Depends what you mean by powerful. Territorially? Culturally? Militarily? Politically? A combination of them? Under the Macedonians the Empire may not have had the territorial extent of Maurice for example, but in terms of military, economic and cultural power, it was at it's apogee.
So, my suggestions of POD's are as follows:
1. Justinian I does not reign as long, resulting in overextending and practically bankrupting the Empire.
2. Maurice pays the army and is not executed by Phocas with all the chaos the Tyrant's reign subsequently brought to the Empire.
3. Heraclius wins the Battle of Yarmuk, stemming the Arab tide, and securing Byzantine power in the Levant and perhaps preventing the fall of Egypt.
4. Justinian II Rhinotmetos doesn't concentrate on revenge and uses his undoubted skills to good effect to reconquer lost territory and continue his development of the
themata. For a TL in progress about this go
here.
5. Constantine V Copronymus doesn't die at an inopportune moment. Butterflies affect whether Irene ever happens.
6. John I Tzimisces reigns for considerably longer.
7. Basil II Bulgaroctonus fathers an heir who only needs to have half his abilities.
8. Isaac I Comnenus doesn't pass on the Empire to Constantine X Ducas, instead passing it on to his brother John. Alternatively, he reigns for much longer, perhaps passing the Empire on to his nephew Alexius.
9. Romanus IV Diogenes wins at Manzikert (probably the last
decent chance to change things so that you have a powerful Byzantium in 1453).
10. John II Calajohannes lives longer. Alternatively his other son, Isaac, becomes
basileus instead of Manuel, whose pro-western policies and tendency towards insane largesse caused problems for the Empire later on.
11. No sack of Constantinople in 1204 by the Fourth Crusade. (this is almost certainly the last chance to do anything useful and have some sort of miracle of a renaissance of power somehow).
Sargon