I think neither would be truly more powerful. Each power has its own limitations. The Death Note requires a name and a face to be effective, and the Geass has limits on how often it can be used and its range. In a crossover, it would likely come down to who can outwit the other. If Light can figure out Lelouch's identity and write his name quickly, he wins. But if Lelouch can use his Geass to make Light expose himself or give up the Death Note, then he wins.
It's hard to say who would be the most powerful. Light has the power of the Death Note which can kill anyone directly. However, Lelouch has the Geass which can be used in very strategic ways to manipulate people. In a direct confrontation, if Light knew Lelouch's real name, he could write it in the Death Note. But if Lelouch could use his Geass on Light before that, he could make Light do things against his will.
In a crossover, the two different power systems would create a really interesting dynamic. The Death Note's power to kill anyone whose name is written in it, and the Geass which can control others' actions. If Light and Lelouch were to cross paths, there would be a battle of wits as they both are highly intelligent characters. Light would be cautious of Lelouch's Geass, and Lelouch would be wary of the Death Note's invisible and deadly power.
It's a tough call, but it could be Light. If he manages to get his hands on the knowledge of the Inuyasha world and its rules, he could use the Death Note very effectively. He could target demons by learning their names, and since many demons in the Inuyasha world are vulnerable to human - like weaknesses in some ways, his strategic mind could make him a very powerful force. He could even manipulate some of the human characters in the Inuyasha world to do his bidding.
Lelouch would be really interesting. His complex personality and his ability to manipulate with Geass would make for some great interactions with Naruto. He could try to use Geass on Naruto, but Naruto's strong will might resist it, leading to a power struggle of sorts.
Another possible antagonist could be the CCG in the 'Tokyo Ghoul' universe. If they were to somehow cross paths with the 'Death Note' world, they might see the power of the Death Note as a threat and try to take it. This would put them in direct conflict with both Kaneki and Light, as Light wouldn't want to lose his tool for 'justice' and Kaneki wouldn't want the CCG to gain more power to hunt ghouls. So, in this complex crossover situation, the CCG could end up being an antagonist.
Lelouch is quite popular. His intelligence and strategic mind could be very interesting in the Harry Potter world. He could use his skills to outwit Voldemort or manipulate the Ministry of Magic. Another popular one is Suzaku. His sense of justice could be explored in the context of Hogwarts' houses and their values.
If it was a crossover with 'Bleach', Ichigo would be a great choice. His role as a Soul Reaper with his own set of powers could mix well with the world of 'Code Geass'. He might be resistant to Lelouch's Geass due to his strong will, and they could either end up as allies or opponents depending on the situation in the fanfic.
Naruto himself would be very interesting. His naive yet pure-hearted nature would be in direct contrast to the dark power of the Death Note. He would struggle with the moral implications of using it, and it would be fascinating to see how he would try to find a way to use it for good, if at all.
In such a fanfiction, the story could start with Naruto being summoned to the Code Geass world by some strange magic or technology. He'd be confused at first, but then quickly adapt. The Akatsuki could also be brought into the story as a sort of anti - Britannia group. And the Geass users would be both intrigued and cautious of Naruto's power. There could be epic battles between the ninja and the mecha, and lots of drama as the two different cultures collide.
If Naruto got the Death Note, it would be a complete chaos. Naruto has a strong sense of justice, but his understanding of it is very different from Light's in Death Note. He might initially be horrified by the power of the Death Note and try to destroy it. However, if he decided to use it, he could potentially target powerful enemies like Orochimaru or Pain without having to engage in direct combat. But this would also corrupt his ninja way as killing without a fair fight goes against his beliefs. He would likely struggle with the moral implications throughout the story.
The relationship between C.C. and Rukia would be quite complex. C.C. has a very mysterious and detached personality, while Rukia is more duty - bound and has a strong sense of camaraderie. They could start off as rivals, both being strong female characters with unique powers. However, as they get to know each other's pasts and motives, they might develop a grudging respect. C.C. might be intrigued by Rukia's connection to the Soul Society, and Rukia could be curious about C.C.'s immortality and her role in Lelouch's life.