Saba.

I like their (cultural, historical, etc) background, I like their starting position, I am not even remotely able to use their light units in battle . . err.
No, seriously, I loved their campaign in EB I. If it hadn't been for the Ptolemaioi destroying my empire everytime, it would have been the perfect campaign for me, and be it only for the semi-observing style of the campaign. When I read the EB II Arabia preview and learnt about the new province distribution in Arabia, I was very much tempted to play them already. When I saw the screens of their UI and the campaign map of South Arabia it was decided.

After that I'll most likely return to my old love Carthage (re-reading "Carthage must be destroyed" by Richard Miles at the moment) and then most likely try either the Arevaci or the Boii. Or Pontos. Or Hayasdan. Or Baktria. Or the Getai. Or . . .

Infact I'm very much looking forward to play all of the factions (with the notable exception of the HA nomads, I just am awful using them, I'm an infantry army addict ).