I checked some of my favorites and I think Tearmoon Empire did a great job:

The sky was soaked in the crimson light of the setting sun that glared angrily from the horizon onto the Grand Square of the Imperial City. At the center of this famed square loomed a guillotine, its crude and rusty blade dripping red.

For once it follows the same structure as one of the opening lines I spoke of in my earlier post that immediately hooked me:

“The sky above the port was the color of television, tuned to a dead channel.” (from Neuromancer, William Gibson)

Just as the sky is usually not looking like static, the sun shining down on something is usually associated with something positive, but here the sun is angry. Makes you immediately think that something is wrong.

And if the "angrily" in the first line wasn't enough to instill a sense of foreboding in the reader, the next line about the guillotine with the red blade is the point where I sit up a bit straighter and want to know what's happening. Great opening lines. 10/10

Rather than just say "X book did it best" I thought I'd go through some of the LNs in my collection and see how they handle the first line.

The False Hero

"I'm sorry, please forgive me," a gentle voice pleads.

I know that it's kind of a common trope to have an isekai start with a random voice saying something vague but impactful, but at the time I first read this I was pretty engaged. Who are they? Why had they had to apologize? Why couldn't they say more? How did this relate to the title, "The False Hero." Etc.

Sure it could have been better, but it wasn't so cliche as to turn you off from the book nor was it so generic as "I was doing X and then I was suddenly in Y world" or "I got hit by truck-kun"

6/10

Reincarnated as a Sword

The very first thing that came to mind when I opened my eyes was how dark it was. "Hm. Is it night?"

Okay so this is one of my absolute favorite LNs but this first line is BOOOOORING. Oh, someone woke up and it's dark and they're wondering if it's night. You know, something SUPER normal and mundane. Wake-up starts are cliche as well. Sure it picks up later on but this is a horrible start.

1/10

The Villainess is an SS+ Rank Adventurer

The orchard within the Royal Villa was my home within a home. Undaunted by the tall shadows cast by the walls, the blossoming apple trees and flowering shrubs sung a song of springtime as they gently woke me from my afternoon nap.

Okay, we get a little bit of scene building here and a tiny taste of some personality of the narrator seeing that they like to nap in an orchard, but this opening is even less exciting than the described afternoon nap.

3/10

Failure Frame

We were on a school trip when it happened.

Surprisingly good intro here. It's concise and yet it tells several big things. The narrator is a school student. Something noteworthy happened that they are now going to tell you about. And other people were part of the aforementioned incident. All that in nine words. Sure it's not mind-blowing or a huge attention grabber but it doesn't have to be. It just has to get you to keep reading and it does that.

8/10

Solo-Leveling

[The daily quest has arrived.]

It introduces this as LitRPG but you pretty much could have gathered that from the title-alone. Still, if you aren't familiar with the genre it does an okay job of setting up expectations.

2/10

But this is in an unbelievably short Chapter 0. It doesn't even count as a prologue. And all it does is say "Hey this is LitRPG." So to be a bit more fair, lets look at the real start of the novel on Chapter 1.

E-rank hunter Jinwoo Sung. That title followed him around everywhere.

Now we're talking. This is a great start. We can infer that "E-rank" is a very low rank yet this person is our protagonist. We wonder what a hunter is and does. And finally, we know that Jinwoo Sung has a reputation of being E-rank and therefore weak. How is the weakest person the main character? Well, probably from Solo-Leveling! Lets keep reading!

9/10

Trapped in a Dating Sim

Ideas like good and evil are relative and differ based on point of view.

Based on the title, this LN is clearly a wacky isekai. So why is the narrator waxing poetic? I think it does a good job of subverting the reader's expectations and making them want to read at least a bit more to figure out what's going on here.

7/10

Ascendance of a Bookworm

Urano Motosu loved books.

At first glance this seems like a terrible intro. We already know that the protagonist loves books from the Bookworm title. And yet, I think this is a pretty great opener. Of all the ways this could start, it tells you something that should be obvious. THAT is how much this person loves books. That even putting it in the title is not enough to convey their love.

Also we get the protagonists name here. However, this is an isekai and it's surprising that we get their pre-isekai name first (although the reader doesn't know that yet). A lot of novels start with the main character not remembering their name or something making their past life not particularly relevant. However their name is the first thing you read, implying that the fact they're from another world will play a big part in the story.

7/10

So based on my arbitrary scoring system, Solo-Leveling has the best start!

When I wrote that question, I was thinking about the beginning of two regular novels that had me hooked in the first lines. I thought that until this question came up I would have found some light novels with incredible opening lines, but I completely forgot, and haven't come upon some that were so great to make me remember this question.

But just so this post doesn't stay empty I'll post something that I just came across while playing with a new Myne function. I present you with this masterpiece:

I was nothing but code, a collection of 1s and 0s, until the day they started calling me Myne.