Since Emperor Elagabalus was (allegedly) very effeminate and in fact was alleged to have wanted to be a female, and is one of my favorite emperors/empresses, I would vote for him/her/them if listed. If a list of rulers includes someone who - according to stories about them - was confused about their gender then that ruler might as well be counted in either gender, especially if it makes a funny answer.
It indeed would make for a funny answer, but I suppose I'm more interested in the concept of Roman empress as a socio-political role. Regardless of the possibilities surrounding Elagabalus' personal relationship with gender, he was an Augustus rather than Augusta in the eyes of Roman politics and society.
As someone with a passing interest in the Crisis of the Third Century, might I ask which legions Zenobia administered? I read somewhere that the legions stationed in Palmyra largely stayed out of affairs there.
I have read that too (on the basis that there is no explicit evidence for imperial Roman units serving under Zenobia), but I favour the position of certain scholars (e.g. Udo Hartmann, Nathanael Andrade) that Odainath and Zenobia ought to have had Roman units under their command since it would otherwise be very difficult to explain how they defeated the Persians, defeated Ballista and Quietus, exerted their authority in Syria, invaded the Persian Empire, defeated the expedition of Heraclianus, extended their authority into Arabia, Egypt and Asia Minor, and then put up a fight against Aurelian (putting up a strong fight in the Battle of Emesa). It is possible (and perhaps likely) that Roman units looked to Odainath as a figure of authority soon after the capture of Valerian, since the unprecedented situation had established a power-vacuum. Odainath already appears to have been a man of exceptional power and influence, being the first person in Palmyrene history to be called consularis and exarch. He appears to have had experience in warfare, having been involved in the defence of the Palmyrene caravan trade from Saracen raiders, and was honoured by the Roman imperial government as consularis perhaps for this reason (another option is that he was the governor of Syria Phoenike). His initial victory over the Persians in 260/61 may well have inspired defections to his command, as would have his defeat of the usurper Quietus in 261 and Gallienus' granting of support, including the apparent bestowal of the title of Corrector Totius Orientis ('Corrector of the Entire East') soon afterwards. It should also be borne in mind that, while literary traditions tended to 'other' the Palmyrenes, the traditions having stemmed from writers loyal to Aurelian and his successors, Palmyrenes by this time were Roman citizens. Certainly in 270 the Third Legion Cyrenaica stationed in Arabia was forced into loyalty to the Palmyrene regime following their defeat to Zenobia and the execution or battlefield death of their dux Trassus (whether or not they actually fought for Zenobia afterwards is another question). Andrade (2018: Zenobia: Shooting Star of Palmyra) also points out that the cataphracts who fought for Zenobia at Immae and Emesa were probably imperial Roman rather than local Palmyrene units, since the epigraphic evidence for Palmyra's local soldiers revolve around bowmen and camel riders.
Julia Domna - the story of Caracalla not being able to find any assassin who would take the job to kill her shows how great a woman she was, how loved she was. She already saw Geta killed in her lap by Caracalla.
Zenobia for carrying the rebellion against the Empire. Even though in the end she lost to Aurelian. She was not loved, but she was a most interesting figure.
Theodora and Galla Placidia are always interesting. As is Aggripina the younger, who from what I understand was quite villainous: and probably the inspiration for Atia’s character in HBO’s Rome.