Hard to pick one. They are probably no big deal, but it is so satisfying when they kick in.
I'm probably proudest of the master bath & powder room automations. Each room has both a door open/close sensor AND a motion sensor. If you open the door on a dark room, the light comes on. If somebody else left that door open, and you walk into a dark room, the light comes on. If you close the door on a lighted room, the light stays on indefinitely. If you open the door on a lighted room, you are exiting and the light turns off. If there is no motion for a while in an open room, the light turns off.
My most elaborate is the bedtime automation. First it turns lights on in the rooms we'll pass through or visit, then minute by minute it starts shutting lights down starting with those furthest from the bedroom and eventually leaving the slightest night light glow where we need it.
I'm also super happy with the bubbler fountain automation. We have this fountain along the approach to the house, and we only care to run it when people are present ... and always whenever company arrives. I used to scramble whenever somebody rang the bell, so that the fountain was running whenever I buzzed them into the courtyard. Added an I/O Line controller wired into the entrance gate. NOW whenever that gate opens, the fountain comes on (for 10 minutes).
And this one is super dumb, but ... I just added a Philips Hue LightStrips to the back of my TV. When you turn the TV on, the LightStrip turns on. Turn the TV off, the LightStrips turns off.
Now, if I could just get Indigo to notice whenever my screen goes into screen saver or sleep ... so I can control the lights in that room accordingly.