What is the highest quality source of phosphorus?

Lecithin — is indeed a good source of phospholipids, which contain phosphorus.

Eggs (especially the yolk) are rich in lecithin: about 1.3 g of lecithin in one yolk. By consuming 2–3 eggs a day, a person gets 2.5–4 g of lecithin, which provides a significant part of the need for phospholipids.

However, in general, 1 egg (~60 g) contains about 85–100 mg of phosphorus. With an adult norm of about 700–1000 mg of phosphorus per day — 2–3 eggs provide about 200–300 mg of total phosphorus. That is, 25–30% of the daily norm.