Every other 2h axe is usable on horseback (save the great bardiche).
The great axe is not better than any other 2h axe for use on horseback. The relevant stats for the great axe:
speed rating: 94
weapon length: 96
swing damage: 43 cut
Can't use on horsebackNow, the persian battle axe (more damage, similar length, slower):
speed rating: 90
weapon length: 95
swing damage: 45 cut
The two handed war axe(less damage, similar length, faster):
speed rating: 95
weapon length: 92
swing damage: 40 cut
Bardiche (same damage, longer reach, slower):
speed rating: 91
weapon length: 102
swing damage: 43 cut
Notice that other weapons in the same class have the same or greater damage and approximately the same length.
The only major difference is the speed, and speed doesn't count for shit with 2h weapons on horseback. Note: Remaking this topic with a poll, for the last topic and related discussion, see:
http://forum.c-rpg.net/index.php/topic,10338.msg149462.html