Use whatever works for you. If it's a board that will likely be tested, serviced or repaired by someone else, then use whatever works for them too.
I tend to:
- put GND points on my PCBs such that nowhere on the board is further away from a GND point than the length of the ground lead on a scope probe. For these I use 1mm drilled holes, into which I can solder short wires as needed. Using a through hole means they're strong enough to support the weight of a few probes, and to survive the odd tug.
- put test points on any nets which I think I might possibly want to probe during the bring-up and testing of a board. These are just circular pads on the top (or bottom) layer, 1mm dia. They just need to be big enough to probe with a scope, or to solder a short wire onto. I use very thin (30AWG) wire to avoid transmitting any significant force to the PCB. Care is needed to ensure the wires don't pull the test pads off the board; I'm completely OK with this, but if you have room and prefer the robustness of a through-hole connection, that makes perfect sense too.
If test points will be used with a bed-of-nails jig, then they need to be far enough apart for the jig to be manufactured. It may be helpful if they're on a regular grid.
I rarely use test point components (posts, hooks and the like). I don't like the rigidity; if a scope probe connected to a fixed test point gets knocked, then there's a danger it'll bend the probe hook or damage the PCB. I much prefer a flexible wire soldered into a hole.
It's most important to put TPs on nets that aren't otherwise accessible. BGA and (arguably) QFN pads are the obvious candidates, plus any net which only exists on the opposite side of the PCB to the one normally exposed.
I generally reckon that any pin on an IC with >0.5mm pitch is fair game for being probed, and so is any passive component 0603 or bigger. Service and repair technicians may beg to differ, so you need to make the trade-off between scattering test points everywhere vs actually fitting all the parts onto the PCB. TPs can take up a surprising amount of space on a dense board, especially if they have to be through holes.