Easiest Pattern to Recognize with Machine Vision
In case you can shoot a video of the static scene than a blinking light would be the easiest as you could easily detect it by subtracting the n - 1 frame from the n frame until you see something with high values.
If you take a still shot you can use 2 main ideas:

If the colors of the scene are from a given plate, find a color very different in Hue and make sure it is intense (Even active lighting which will saturated).
Use a pattern which is easily detectable like stripes of B/W or pure green and magenta. You may encode data in the stripes.

Resources
Some resources pasted here in comments:

Fiducials / Fiducial Markers.
AprilTag 3 - A visual fiducial system popular in robotics research.
What the Name of This Kind of Computer Vision? (Related question).