No matter how gifted, you alone cannot change the world. But that's the wonderful thing about this world - L Lawliet.
My answer would be - both. Both Light Yagami and L are extremely well written and well portrayed characters. I loved watching Deathnote, not only these two but the entire cast of characters in this anime/ manga are too beloved by all who've grown some emotion/ attachment towards this anime. In the end however I like most fans would've loved for L to have been kept alive to the very end.. and the same goes for Light Yagami a.k.a. Kira. No one plays exceptional great deductive skills like these two do. Both their portrayals is simply flawless. Kudos to the creator for bringing in such characters to life. L and Light, always forever in our memories.
I'll solve equations with my right hand and write names with my left. I'll take a potato chip... and eat it! - Light Yagami.