Making stocks/sauces. Salt, fat, acid, heat as it is said. If you can master basic stock, you master basic skills: sourcing ingredients cheaply, veg prep, roasting, sautéing, deglazing fond, skimming, clarifying, etc. You can practice reductions/dilutions, you can observe the properties of gelatinisation and see how it changes while making a glace de viande/bouillon.

Next would be the roux in all its stages from white to brown. Teaches you how to clarify butter, how to smell when something is getting nutty vs burnt. Hones one's patience and attention to detail to avoid scotching the product. Opens the door to bechamel, mornay, velotue, espagnole, demi-glace, bordelaise, chasseur, diane, etc.

I'll include two here as technically, the one is historically a roux thickened veloute but not anymore -- emulsions and tomato sauce. Emulsions are fat particles suspended in acids and air, coagulation is denatured protein aka cooked (creme anglaise family): hollandaise, mayonnaise, beurre blanc, vinaigrettes, etc. To even make one you're mastering the use of a whisk and steady pour, bain-marie, reductions, precision heating/cooling. Tomato sauce now is all fat, acid, fruit, veg, aromatics though it used to be salty, meaty, and thickened with flour. Whichever way you cut it, once you know basic tomato sauces and cancasse you can explore creole, marinara, neapolitana, etc.

I think mastering those basically opens every other day...