What a wonderful question Huacanacha. The truth is that it would receive better treatment as a hub (with pictures and videos) than a short answer. But I will try my best.
Regardless of what type of nails you are born with, they should be properly maintained (with a nail file and light sand paper every day, to prevent nicks and scratches).
They need not be long. Indeed, the ideal is to hit the string simultaneously with the flesh of your finger (to dampen the string) and the nail (to produce a larger sound). If your nails are too long they will interfere with your flesh hitting the strings; if your nails are too short only the flesh will hit the string.
As for me personally, my nails are very soft and break too easily. So playing with them was not enjoyable. So I think that the real answer to your question is "strong nails are the best to play classical guitar with."
Also, there are some issues with shape of the nail (flat, arched, dip), etc. But that is not easy to discuss outside of a hub, where you can have multiple images.