Alas I do not have a favourite emperor.
Though the following is the top five of the highest number of AE3/4 coins from the reign of an emperor that I ended up buying last year, i.e. a years worth.
This shows who is the most commonest emperor to come across in AE3/4 coins.
1) Constantius II: 160 coins bought
2) Valens: 79 coins bought
3) Constantine I: 61 coins bought
4) Constans: 54 coins bought
5) Valentinian: 51 coins bought
Suffice to say I am sick of the sight of all of these emperors AE3/4 coins.

The emperors who seem to sell well, are the likes of Titus, Domitian, Caracalla, Elagabalus, Probus, very short reigned Emperors and Caesars and 5th century western and eastern Emperors.