Geass: C.C. suddenly give Lelouch the "Power of Kings", the mythical power of Geass. The Geass manifests itself in him as the power of absolute obedience, which allows him to make people obey his orders without question. Each geass user has one unique power. Lelouch's father had the power to erase memory.
Sharingan: Is the blood inheritance limit of the Uchiha Clan. The ultimate form is the Mangekyou Sharingan. Each user develops their own unique looking Mangekyou comas. "With this strength it was said Madara used Sharingan's normal ability to suppress the Kyuubi and tame it." There are 3 Mangekyou techniques are Amaterasu(Black Flame), Tsukuyomi and Susanoo.
To compare it to Geass we will discuss the Tsukuyomi technique wjich is generated by the left Mangekyou eye. It is a Genjutsu technique which uses the Mangekyou form to initiate a mentally and physically harmful illusion. To utilize the illusion, the clan member will look into the eyes of his opponent and cast a Genjutsu spell on them. The spell takes place in the mind of the one affected, while the jutsu is cast the one it was cast upon is at the total mercy of the Uchiha clan member. The clan member has full control over space and time in the jutsu and can place the one affected through any hell they desire for any length desired. The technique itself only lasts for but a second in reality, but the one affected could have live through it for 7 days of pain. Only a natural born member of the Uchiha Clan can is said to be able to defeat it and even then its hard. Like when Itachi and Sasuke where having a fought purely out of Genjutsu they both struggled.
Death Note:
A supernatural notebook used by the Gods of Death. If the user writes down the name of a person into the notebook while picturing that person's face in his mind, that person will die. After a name is written, the user has forty seconds to write down the cause of death, and then another six minutes and forty seconds to write the details of the death. If no cause is specified or the cause and/or details are impossible, the victim will die of a heart attack.
To compare it with Geass, the Death Note is limited, You must Know the victims correct name for it to work. You're not going to walk up to some one and ask them whats is your full name so I can kill you. Took Light forever to just Kill L. It's a note book so it can be stolen and used against you. It is time restricted to minutes compared forever to for geass and days for sharingan. It is the worst weapon of the three.
Final review:
Geass is king in the lasting effect for the effect can last almost forever depending on the circumstances. Sharingan Temporary and Death Note hasty. The Geass is forever and cant be taken away, a uchia can have their eys taken away and give non uchiha powers like kikashi or a uchiha like Maderia stronger. Ounce again a the Death book can be stolen, damaged or burnt! Effective liberation Geass is king due to its last effect. You can have they killed in anyway possible or make them your lackey. Sharingan is king of killing for it has multply abilites where as Geass users and Death Note only have one.
The most best:
1)Geass/Sharingan
2) Death Note
Any question or disputes etc..please comment.