A quick little something that needs saying:
The question is asking which of the 3 listed characters is "best"; either by personal preference, some logical figuring of why one is better, or just based off of some random thing that just touches you the right way. It's only asking about the faceless (up untill a point) voices that just came over a computer to solve the cases that nobody else could. It isn't a question as to which character, out of all the characters in the show/manga is best, only of these 3, who have something in common. It isn't just the initial to their first name (but, ironically enough, it is that)
L is L, and only L, as M in Mello, and N is Near. That's it, nobody else as far as this particular topic goes.
/mini-rant xp