A really good completed light novel is 'The Melancholy of Haruhi Suzumiya'. It has a unique premise where the title character has the power to unknowingly change the world around her. It's filled with humor, interesting characters like Kyon, and has a cult following. Also, 'Overlord' is great. It follows the adventures of a powerful skeleton lord in a fantasy world, and the power dynamics and political intrigue in the story are very captivating.
Some top completed light novels include 'Re:Zero − Starting Life in Another World'. The time - loop mechanic makes for a really interesting narrative as the main character Subaru has to keep trying to change the outcome of events. And 'No Game No Life' is also excellent. It features a brother - sister duo who are transported to a world where everything is decided by games, and the clever strategies they use to win are very entertaining.