This is the 23rd most frequent Hungarian word.
egyik
"One of" or "either."
In this sentence, "egyik" is used to refer to one item among a group, specifically one friend among others.
Az egyik barátom már itt van.
One of my friends is already here.
Here, "egyik" denotes one option among a selection of the better restaurants.
Nem tudom, melyik az egyik legjobb étterem a városban.
I don't know which is one of the best restaurants in the city.
In this context, "egyik" refers to an unspecified specific day among several.