Depends exactly what you’re asking about. Greebles specifically? Or just parts as parts….
For an all-around universal part, I find the 1×1 brick with studs on all sides to be essential. I use at least one in nearly every MOC. I can’t really imagine building without it.
For surface details, I don’t have a single Universal Greeble, but I do have a small suite of Go-To Greebles. I find that if I start with 1×1 round plates, 1×1 round tiles, bars 4L (lightsaber blades), lightsaber handles, 1×2 grille tiles, and 1×1 clip top tiles, I can greeble effectively for quite a while before I need to mix in other parts. Second tier favorites include ribbed hoses, taps, pneumatic tees, 1×1 plates, 1×2 jumper plates, headlight bricks, and grated cheese. After that maybe it’s time for some 2×2 round tiles, 2×2 tiles, 2×2 jumper plates, and 2×2 radar dishes. That’s usually enough parts variety for solid generic greebling…though it’s generally better to half-design half-functional-looking pseudo-subsystems than to just spray dots and lines everywhere.
If I had to pick just one greeble part, I think I’d agree with the 1×1 round tile as both essential and versatile.
As for the “ILM Universal Greeblie” in the link…it doesn’t seem all that universal to me…I think the closest LEGO equivalent is the 33291 Plate, Round 1 x 1 with Flower Edge (4 Knobs). OK, I guess if that came in gray I’d use it pretty often.