Rodinal for slow and medium speed films (50-100), specifically PanF+, Acros and TMax 100. Parodinal is a clone made from paracetamol (acetaminophen), and works just the same.
HC-110 for ISO 400 films. No clone available, and chemicals are rather difficult to obtain. The exact formula is also a trade secret.
Some swear by Ilford DD-X. It is expensive compared to the above alternatives. I think it is a PQ developer, and could be replicated if you can get hold of Dimezone S and hydroquinone. But don't hold me to that one.
There are a few "open-source" options, e.g. PC-TEA, but only if you know what you are doing when working with chemicals, and can source the raw materials.
If I had to settle on only one, it would be HC-110, used as single-shot dilution H (1:64). However, it seems to be going up and down in availability. B&H currently lists it as discontinued, don't know if that is permanent or not. A 500 ml bottle will develop a hoard of film, so it may be all you need for three or more years. I am also using TMax Developer quite often, and it is a rather easy and excellent developer overall. It is great at shadow detail, and worth having for that reason alone. I tend to think that it is not possible to combine fine grain with high acutance and good shadow detail. If you pick two, the other one will suffer. For slow and fine-grained medium speed films such as Acros, no matter which developer you use the grain will be fine enough. So it makes sense to choose the developer that gives you the best tone curve for what you typically want to photograph, according to your taste.
As you are in Europe, you could maybe find what you are looking for at Maco Direct or one of the other web-based suppliers. We have ordered from Maco to South Africa, and the shipping was quite reasonable.