Light is an unrealistic but very entertaining character, including due to his wit and intelligence.
Tenma is obviously rooted in reality, so it depends on what you look for. If we wanna exaggerate, itยดs like comparing a superhero/super antihero to a real life person.
I myself definitely prefer Tenma, since you can much more empathize with his journey. Light was a finished character almost immediately, and the rest was just a battle of wits between him and L (iirc, Light immediately talks about becoming a god of the new world at the end of episode 1 or 2).
Support characters also goes to Monster, but i maybe biased since a lot of the story was based in Germany, and the characters were really realistic.
Monster better plot, especially since it does not overstay its welcome, DN unfortunately does (Lยดs death was supposed to be the end, mangaka duo got "forced" to extend the story).