There are two components to your questions:
- singular or plural?
- which article to use: "the" or "a"?
1. Singular or plural?
This rule is useful to remember:
"one of the (many) items"
If it is one of many, then "items" always has to be in the plural. Why? It is because we don't say "It is one of one item"!
So that answers your question about the singular "member". We say, "one of the most massive members of the vegetable kingdom".
2. "The" or "a"
(a) one of the [plural noun]
· one of the daughters
· one of the managers
· one of the ugliest girls on the room
· one of the richest men in Russia
· It is one of the largest members of the animal kingdom.
(b) a member of
· He is a member of the Carlton Club.
· Mary is a member of the hockey team.
· His mother is a Vanderbilt. Gloria is a member of the Vanderbilt family.
· It is a member of the animal kingdom.