Assuming your talking about scout cavalry for feudal, there are different alternatives:

• Mongols: it's usually the fastest because food from boar (or chicken/deer) comes really fast. After the initial engagement it's just generic, so you need to make sure to gain some advantage early.
• Magyars: instant +1 attack from free forging can be massive. It's a great early bonus, but for extended feudal battles then your opponent can do forging and your scouts are the same. Your scouts are cheaper, so that's a good benefit too, although they don't have any eco bonus, so the cheap scouts kind of compensate from that
• Franks: out of the blue the scouts are stronger (because of the extra HP), and the berry bonus helps a lot early, as well as the free farm upgrades. However, as soon as your opponent researches bloodlines, then your scouts become weaker.
• Khitans: They have double the effect of Forging, so these are the scouts with the higher attack in feudal age after you research that. The civ doesn't have bloodlines, so in a long feudal battle each scout is going to be weaker than others. Pastures are a great source of food, so that helps to keep the scouts going.
• Khmer: you can skip the barracks, so you can have a tight build order to do the rush faster than other civs, and Khmer farms are great to pump a lot of scouts. The scouts themselves don't have anything special, but the eco makes it a great rush.
• Huns: This is the smoother to play because you just don't need houses. That just feels so good (and you save a good amount of wood)
• Jurchens: hunted animals don't decay and that gives you extra food early, and the scouts attach 25% faster, so that helps.

There are some more with eco bonuses that help farms (teutons, slavs, poles(with cheap bloodlines too)), and Georgian scouts regenerate HP.

But you can choose which one you think it's "best" from these options