There are many types of Geass, but I'll focus on Lelouch's Geass.
Geass usage has no limits, however, it can only be used one on a specific person.
Geass is like Koto Amatsukami. When used on a person, a command must be given, and that person will follow follow that command absolutely.
KotoAmatsukami can be used once every decade and it's effect is akin to Geass. However, if a the person under the control of KA becomes aware that he is under its control, KA's effects will be neutralized.
Once a person is affected by Geass, he will remain under its control with regards to its command. For example: If a Geass user gives the command, "Slap yourself every morning.", such person will slap himself every morning for the rest of his life.
A victim of KA is aware of his movements, and has consciousness, but he is not aware of the fact that he is under the control of the user, and if he notices that he is under the control of another person, the user will lose his grip over the victim. When the Kage became aware that Danzo is controlling them, Danzo lost his control over the Kage.
Unlike KA, when a command under the effect of Geass is being done by the victim, he is put in a trance wherein he is no control or consciousness of his movements.
If a victim of Geass is commanded to kill himself, he is put in a trance wherein he has no control or consciousness, and he will definitely kill himself.
In short, KA= Conscious obedience
Geass' effect = Unconscious absolute Obedience
Geass > KA
Geass > Sharingan
But Mangekyo Sharingan, I cannot say the same.