There are too many to list, and it depends entirely on the crop system you're trying to support. For example, for annual crops it would be different than for multi-year agroforestry systems, and you may want some plants to provide windbreak or shade for others at an early stage, and then remove them when others are more mature, etc. Also, you want a mix including some legumes for their nitrogen fixing ability. And some plants are better at utilizing and accumulating specific minerals (bioaccumulators), so may want some of those.

In short, it's complex and requires a good deal of knowledge or willingness to experiment, which is why it's not more widely practiced, even though when done correctly it undoubtedly is very effective, sustainable, and requires far fewer inputs than any other agricultural practices. At least in theory, a master of this art can design systems that are planted once and the produce different crops for several years while mimicking the natural succession of a grasslands in transition to becoming a forest with a fairly small amount of maintenance (harvesting crops at different stages and chop-and-drop'ing mulch plants). One master of this art here in Brazil, whom I've learned a lot from, is Ernest Götsch[0][1].

[0] https://agendagotsch.com/en/

[1] https://en.wikipedia.org/wiki/Ernst_Gotsch