Still one of my favorites

Rating: 9/10 (19 November 2019)

This is the only film I've ever seen 5 times in theaters. I saw Gladiator 4 times (once while traveling overseas). Everything else has been 3 or less. More than 15 years later and it's still one of my favorites.
The real reason this film works is because it's a character drama first and a thriller second. The supernatural aspects are just window dressing. Like The Sixth Sense and Unbreakable, M. Night's best films work because you care about the characters and are drawn into their stories. He basically tricks audiences into eating their veggies; you're watching character dramas about human beings struggling with very real human issues. Usually those sorts of films don't exactly light up the box office. The genre supernatural stuff is just the cherry on top.
With that said, it amazes me that people still lambast this film for plot holes that *aren't* actually plot holes. This film is very well thought out, and all of the little details that are put into it do in fact add up to something. I'll have to venture into spoiler territory now...
***SPOILERS FOR THE SIXTH SENSE, UNBREAKABLE AND SIGNS***
M. Night films are know for their twists. Yes, he's almost ventured into parody territory over the years with these surprise endings, but when they work, they work exceptionally well. Signs I think has the most subtle twist, and in many ways it is his most powerful ending.
Essentially, M. Night uses genre tropes to draw you into a world, and then he flips those tropes around. So in The Sixth Sense, you think you're watching a movie about scary ghosts, and then at the end you realize you've been watching it mostly from the perspective of the ghost. In Unbreakable, you realize at the end that you've been watching a superhero origin story all along. Signs follows a similar pattern. You think it's a story about an alien invasion, but then at the end you realize you've actually been watching a movie about the apocalypse. And like Unbreakable, the film isn't really shy about letting you know what is really going on. Unbreakable has comicbooks as a major aspect of the storyline before the reveal, just as Signs has religion/faith/God as major story points before the reveal.
I'm not the only person who has figured this out, mind you. You can google "Signs demon theory" and plenty of other people point out the clues (er, signs?) throughout the film. Which brings me to my original point: water doesn't kill the aliens. There's no plot hole here. They aren't aliens. And it's not just water, it's holy water specifically that burns and kills the demon in the ending scene. Yes, you might think it's cheesy, but I think the movie earns it. You care about the characters. You care about their state of mind. The entire story builds up to this very specific moment.
And just in case you aren't sold or think I'm trying to write M. Night out of a hole, keep this in mind: Bo is referred to as "like an angel". The water that kills the demon is her water, it's not just any water. More importantly, the TV mentions that a primitive method has been discovered in "three Middle Eastern cities." Last I checked, the Middle East wasn't exactly know for an abundance of water. However, it is the birthplace of the three major Abrahamic religions, and the River Jordan is even considered holy. If it was just any old water that killed the "aliens" wouldn't a solution be found first in Seattle or Mawsynram or something?
The fact that they're demons is even more obvious when you think of the fact that they look demonic (with their cloven feet, etc.) and are running around naked. Even the crop signs themselves look like a pitchfork, which is admittedly maybe a bit too much on the nose. You also never see a spaceship once. This is purposeful from a writing and directing standpoint. Lights in the sky could reference portals or gateways. It has nothing to do with interstellar space travel, that's just something the characters and audience infer before they have all of the facts. The only time the word "alien" is actually mentioned is when they're wearing tinfoil hats. That joke isn't for nothing!
Keep in mind, M. Night was raised as a Hindu himself (but went to Catholic school) and Signs in many ways plays with tropes from both Christianity and Hinduism. Think of the asuras, powerful superhuman demigods in Hinduism who are often considered "enemies of the gods". Wasn't there even a line in Batman v Superman about demons coming from the sky? Signs is an apocalypse story in the same way that Unbreakable is a superhero origin story or Split is an Unbreakable spinoff/sequel. You don't realize it until the end when all the pieces of the puzzle come together.
So TL;DR version: Signs is a great character drama about the apocalypse (remember: apocalypse means revelation). A family struggles with grief and has to fend off demons during humanity's darkest hour. There aren't any gaping plot holes, either. Water doesn't kill aliens, but rather holy water kills demons. This film is *so much* better than dreck like Left Behind, too. If you watched Signs years ago and hated it, give it another try!