It all depends on map and players in enemy team.
The best thing is to have several horses to choose from.
This is true no matter if you play 1h, polearm or ranged.
The arabian horse can be godlike or total crap depending on the above.
A destrier is always somewhat decent on every map so if you plan to heirloom one you should maybe pick the destrier. It's the best all around horse.
Being a lancer on a "ferrari" myself I often end up running straight into a wall or other obstacle simply because the bad maneuverability of the courser. I really hate it when that happens and wish I had heirloomed an arabian horse instead.
Well... now I'm about to fully heirloom the Arabian too
After that I might heirloom the Destrier. So what horse is the best really depends on the situation.