What are your three automations you are fond of?

flightradar notification on my smart tv so i know which fucking chopper or plane is hovering around the place i live. i always see a nice picture and their destinations. 1.1. basically all notifications on my tv are nice...like doorbell, fridge left open etc..
morning fan. i turn on the fans weekdays 30minutes before i get up...better distribution of temperature in summer and winter. had much more effect than anticipated.(10$ tasmotas)
all automations HA cant do I use n8n. so when there is a recall for food (like they found some contamination) i push on a kindle display in the kitchen. (like grab data, grab product image, render kindle friendly presentation)

I don't have a lot of automations, but I collect a lot of data with my home assistant. My favorite ones are mostly outside lights.
I turn on my driveway lights at dusk and off at dawn.
Similarly, I do my holiday lights the same, except turn them off at 11 PM.
I have another that alerts me when my ZFS Pool is unhealthy.
Another is to notify when the washer and dryer are finished running.
- I have a washer notification too (dryer beeps on its own). Very handy.
  
  Ours both beep, but can be tricky to hear sometimes. I also have them notify on the television using the Android TV Notification Service. It can be handy when we have it on and waiting for it to finish.

Occasionally someone (me mostly) will forget to hit the start button on the washer only to come back an hour later to a washer full of dirty clothes. An automation watches for the power draw on the washer to remain above the "powered off" but below the "running" wattage for a few minutes and sends a "Washer not started" notification if that occurs.
Roku loves to push menu updates while the TV is turned off and we would randomly find a new menu entry for "Zombie Housewives of Beverly Hills" when turning on the TV. An automation turns on OpenWRT firewall rules blocking Internet access when the TV's are turned off, and automatically disables those rules when the TV's are turned on. Menu updates still happen but they don't seem to happen as often and the automation works so well no one notices it. It also blocks the constant stream of data Roku sends to the company's servers except when the TV's are actually in use. Roku has voice activated remotes that are probably always listening (no matter what the company says) and it blocks that traffic too. Between that and Adguard Home's blocking of all other UI ads the TV's are almost completely deshittified.
I have a couple of old but high quality pieces of audio gear that are used with Spotify Connect. An automation powers on whatever amp / receiver Spotify is playing on using an IR blaster or wifi smart plug and powers them off when play stops. One receiver draws 80 watts when powered on and it was annoying when it was accidentally left on for a couple of days. This automation alone saves us a whole dollar or two a year.
Bonus 4th automation: I bought some Kasa smart bulbs that are really unreliable and randomly will stop responding to wifi commands. Usually 1 bulb out of the 3 in a ceiling fixture will remain on when an automation had sent a command to turn them all off. 3 different redundancy automations watch for mismatches between bulb states and when the offline bulbs wake up again (usually in 30 seconds or so), it resends the command to sync the bulbs. These automations have made the Kasa bulbs usable when they otherwise would have been trashed.

I added a second switch for the basement lights by the bullhead door. Didn't want to run wires for a real three way circuit, so I just put a smart switch attached to nothing by the bullhead and wrote a very basic automation to sync its state and the state of the switch really controlling the lights.

I use a few timers, especially during the holidays. My favorite though is probably tying motion sensors to lights; we have them in the basement, our sunroom, and our bedroom closets. In terms if full automations, I have a wake up routine I built in Node Red the slowly turns on the bedroom lights and shifts them from red to white as a sunrise alarm. It works really well now that I've gotten good lights and tweaked the automation over the last year or so. It takes into account holidays to not activate on days we have off.

I have a notification for when my SO leaves specific lights on for too long

I use X10 for all lights. It's an old standard getting harder and harder to get, but it's just dumb enough to not be a security concern and smart enough to switch off all lights from a remote in my bedroom.

I don't have many, but I'll share my most used one:
My lamps and overhead lights are all smart bulbs. I've got several different modes (lamps only, overhead only, all lights, etc) and can control them with a unified dimmer.
The problem I had at first was that the overhead lights overwhelmed the lamps, and everything was too bright. So I added a step to the dimmer control to set the overheads to 75% of the dimmer value while keeping the lamps 1:1. Works great. I can even toggle that on and off.
Using HomeAssistant with all the logic in NodeRED.