Ackbar is flexible and works with any and all ships. I run Ackbar as commander but I think its the worst. Mace and Tarkin are better. Mace outclasses Ackbar by miles with the right team composition which is something a lot of people lack. Mace needs a lot of tankier ships like the clones which not a lot of people have and run. You can regenerate the large protection pool and keep switching the taunt around and leave Ackbar-led ships into a stalemate or beat them. Mace does really good in defense so its easier to hold rank.
The most hardest and best capital ship would be Tarkin. Empire ships have amazing synergy with Tarkin and they just keep going forever with the TM manipulation with FOTP and TIE fighter while hitting hard as hell. You can throw in a few other republic ships or a third affiliation / synergy with Tarkin-lead and still do good. I think Tarkin and Mace are pretty evenly match when it comes to which is better.
