depends on what you consider 'best.' If you're talking from a strictly technical point of view, trying to maximize enemy losses while minimizing your own, I'll give it to you. Well, actually, let me change that... cheating to give yourself tons of health and uber strength would probably be the best case for that, but horse archery is quite possibly second best.
I prefer to look at things from an enjoyment perspective... I play the characters that give me the most enjoyment. While the feeling of plugging unsuspecting enemies from horseback 100 yards away certainly isn't unenjoyable for me, it does seem to lose its charm after a few hundred kills. For me, it's the lack of challenge and danger. Frankly, its easy mode, and I don't see the point of playing a game without the possibility of losing.
For me at the moment (it changes pretty much weekly) the class of choice is unmounted lightly armored 1.5 hander/shield/bow or javelins, using NPC mod to have 8 heavily armed and armored foot companions. Absolutely nothing in horse archery compares to facing down a cavalry charge with your companions, dodging lances and cutting horses down left and right. The battle is so much bigger and more personal when you're not some aloof god of war, smiting everyone without fear of retribution. The satisfaction of taking out that last knight, with only 2 hp left, completely red with the blood of horse and man alike... well, it's why i still play this game after nearly 2 years.
But that's me. If you get the most enjoyment out of horse archery, then it's the best for you, and that's the beauty of the game.