In a vacuum, it doesn't really matter. Here's the issue, intelligence can be considered, defined, and applied in a variety of different and accepted ways. The efficiency and effectiveness of recognizing and identifying patterns, being able to create detailed and accurate predictions, being able to read a situation with complicated variables well, being able to perform extremely difficult tasks with relative ease, using perception, analytical abilities. Having an 'intuitive' level of skill and understanding of a field far beyond your peers, and so on... intelligence can be a large advantage, but there are other factors it interacts with that are important in reality and sometimes get overlooked in fiction.
Imagine if through whatever means, we discovered that Matsuda, and Mikami were actually both objectively the most intelligent Death Note characters? It wouldn't really make sense right? It would feel... wrong or in error. Except what if Matsuda was too nervous, and anxious to ever really feel comfortable enough to relax and apply his intelligence and what if Mikami was too volatile emotionally, to be objective enough to actually apply his intelligence.
The way I watched and read the anime, I felt like L was more intelligent. He had the tougher task, Light was aided by a supernatural force. Supernatural forces, especially those that we have little to no information of, seriously wreck with our abilities to analyze, assess and predict for and around. We know this from philosophy, epistemology, science... supernatural forces create too many problems and open too many doors, which is why they are often ignored or require evidence before being considered. You can't really compare a team of police officers to a supernatural entity and supernatural book. One of those greatly empowers the abilities, discretion and actions, and the other could potentially require intellectual resources to manage in order to get a little bit of value out of. So I definitely viewed L as having the greater challenge, and it didn't reflect poorly on him, when Light eventually outlasted him. Not only was Light nearly as intelligent, he had access to more powerful tools, that were supernatural and unknown to L!
Also, and this isn't being very objective, but I liked L more. He was more endearing and quirky, Light was respectable and well intentioned at times, but he was also a bit sociopathic at times, and also horrible naive to the way of the world for someone intelligent. We like to favor certain characters in certain ways, but without the pretense of thinking its because we favor them... so I was a little bit surprised when I read a supplement book, where the creator explains that Light is more intelligent. I mean, I am generally pretty critical of intelligent characters in fiction, because unless the writer is very intelligent and... or knows how to write around intelligent characters in such a way as to not reveal its superficiality, such characters end up cliche or inadequate.
I don't think thats what happened in Death Note though, though I do find it curious Light was assigned a 10 in Social Skills, because while he does have generally good social skills, he feigns empathy, rather than applying it sincerely, and this by itself can lead to poor relationships... but thats another topic. Anyway, intelligence in a vacuum, so why did L do so well contextually? Doesn't have to be that complicated, he could have just been more considerate/reflective. Worked relatively harder. In real life, those with the highest IQ's aren't synonymous with those who create the greatest inventions, or who are the most successful in fields... great intelligence is often associated with the latter types of people, its definitely an asset, but if we view it as a tool, well some people work harder to apply a tool than others. I think this reflects well on both L and Light. Both had large ego's and confidence, but I think L was a lil crazier, as far as how much he sacrificed socially, in order to channel his wits. So maybe he just pushed himself a little bit harder... which makes it more tragic, when he fell short... but also sweeter, when he eventually 'won' because of his two successors.
TL;DR: I think Light is more intelligent, but not really by much. L makes up for it in other ways, and also demonstrates it/applies better in certain ways, L also in certain ways. Also who is more intelligent between John von Neumann and Richard Feynman? Ramanujan or Einstein? Where does John Nash fit in? All of them are considered and fulfill criteria around genius, but they also all had different personalities, and characteristics that would go on to determine how much they would accomplish, influence, and intellectually explore, respectively. Also, I just listed great intellectuals from the past, scientists/mathematicians, I am not that familiar with extraordinarily intelligent Detectives and Criminals but that would introduce a large variable into a discussion, and require an extra set of qualities that could benefit the whole.