Photo eyes. Small, cheap, and somehow behind a shocking number of 2 a.m. phone calls. Most of the time the sensor's fine - it's just the wrong type for the job. So here's what I'd reach for, sorted by whatever problem is staring you in the face.
"I just need to know a box is there."
Plain boxes on a conveyor and you can only get to one side? Background suppression. It looks out to a distance you set and ignores everything past that, so the far guide rail or a shiny machine frame won't fool it. For one-sided presence detection, it's my go-to.
"It keeps missing dark parts."
Classic. That's a plain diffuse sensor starving for light - black rubber and dark plastic just don't bounce much back. Swap to background suppression, which cares a lot less about color. Can you get to both sides? Through-beam's even better.
"It has to work every single time, and it's filthy over here."
Through-beam. Transmitter on one side, receiver on the other, part breaks the beam. Most reliable setup there is, and it shrugs off dust and mist better than anything because it's got the most margin to burn (sensor folks call it excess gain). The price you pay: two devices to mount and wire.
"I can only wire one side, but I can stick a reflector across."
Retroreflective. Shiny parts? Get a polarized model. Otherwise that shiny part bounces light right back, looks just like the reflector, and sails on by unseen.
"It can't see clear bottles or shrink-wrap."
There are clear-object retroreflective sensors made for exactly this. They watch for the tiny dip in light a clear part causes instead of waiting for the beam to get fully blocked. A regular retro here will slowly drive you up the wall.
"The part is tiny, or I need to catch an edge."
Laser. Small, tight spot. Overkill for plain old presence detection, though.
"There's no room...or it's hot as blazes."
Fiber optics. The electronics live in an amplifier a few feet away in a little box, and skinny fiber cables run out to the spot. Handy when you've got a bunch of sensing points crammed together, too.
Whatever you pick, a few quick sanity checks. Match PNP or NPN to your input card. Decide light-on or dark-on so the output means what your logic thinks it means. Fast parts? Compare the sensor's response time to how long the part actually sits in the beam. And if you've got the option, go IO-Link. Changing a setting or catching a dirty-lens warning from the panel beats climbing a ladder with a tiny screwdriver in your teeth.
Oh - and none of these are safety devices. Keeping people away from a hazard takes safety-rated light curtains or scanners, set up by someone who knows the standards. No shortcuts there.