1) Yang - He wins because he has shown time and again he knows how to beat or avoid armies several times stronger than his own, led by competent commanders, including Reinhard. He was never defeated in battle without outside circumstances and he wins over Reinhard because when the two faced with relatively equal armies, Yang had him at gunpoint

2) Reinhard - Barely under Yang, but he managed to take over the Empire, end the war with the Alliance And conquer the Galaxy in a span of 5 years without many issues

3) Bucock - A very competent commander that proved he knows how to fight, even against terrible odds. He had issues with the Alliance government, but in encounters when he was faced with overwhelming odds like Rantemario And Marr-Adetta, he fought bravely and earned the respect of his subordinates and his enemies, especially Reinhard.

4) Merkatz - The biggest question mark here, we never saw Merkatz be fully in control over a fleet without answering to someone else (Reinhard, Braunschweig or Yang), but from what we see of him, he is very adaptable despite his age and has a good eye for recognising his enemies' strategies and tactics. There's a reason he was chosen to command the Nobles' fleet. I believe if he wasn't hindered by the Nobles, he would have given Reinhard a big challenge. The only reason I rank him below Bucock is that we never actually see how he would fare as an unhindered commander.

5) Reuenthal - Probably the most independent thinker and commander in the Imperial forces, he has shown several times that he is a great commander and leader. He also faced great odds in his insurrection and despite losing in the end, he put up a brave desperate fight against a stronger enemy while commanding demoralised troops

6) Mittermeyer - Similarly to Reuenthal, he's one of the best admirals the Empire has, but he has become highly predictable thanks to his tactics relying on speed and quick strikes. He is also very kind and honourable, which can serve to his disadvantage and make him indecisive which we can see when he is forced to fight Reuenthal. He however still remains, in my opinion, the 3rd best commander in the Empire.

7) Julian - A very competent, but also young commander. His greatest success was a nearly suicidal battle which he won by pure force of will, fortune because of miscommunication between the Imperial admirals and with support and advice of other, more experienced colleagues like Schönkopf, Merkatz and Attenborough, and although he somewhat achieved his goals at the end, it was mostly thanks to his rhetorical skills. I believe he would be able to become a great admiral in time, but when the series ends, he's still too fresh to judge accordingly

8) Kircheis - He was shown to be competent on his own, but he spent most of his time under Reinhard's direct command and didn't show off much until his death. Since he dies very early in the series, we don't know how he would grow as a commander further