Test points are very important and useful nodes that will make prototyping, debugging and repairing a circuit easier. It is very annoying to try touching a signal with a probe if the node is not reachable. So as a rule of thumb try to add a test point to important signals that you would like to measure for troubleshooting.
There are many ways to implement test points, choose a type according to your needs and space you have on the printed circuit.
A large via or a standard 1 mm diameter pinhole is great for multi-meter and oscilloscope probes. You can use this type of test point for low-speed signals and power signals.
The ones that form a loop are great for oscilloscope probes with hooks, if you will measure a high-speed signal for a long time or measure multiple signals simultaneously this is the way to go. Also, this kind of test points is the best way to observe logic level communication channels.