I am sure you will get lots of different recommendations. Personally, I use Ilford ID-11. Kodak D76 is virtually the same but can be fierce if you mix it and use it straight away - good idea to leave it overnight. ID-11 and D76 become more active if stored correctly for long periods unused. There are many many exotic developers which I do not use but others swear by. Agfa Rodinal has the supreme virtue that it keeps even in part-used bottles seemingly forever, some people like the tight grain pattern it produces, like other developers it gives a compensating effect at high dilutions but may not give full ISO speed with certain films. If I could not buy any ready-mixed developer and had to mix one from raw chemicals, I would probably make some Ilford ID-2, which is great for sheet film but has not been available ready-made for a long time.