Learn Farsi: این یکی بلندترین همه است.

To say the most or least of a specific group, Farsi connects the superlative adjective, like بلندترین, directly to the group it is being compared against, which is همه in this sentence. You link them with an Ezafe. So بلندترین همه with the unseen -e sound literally forms tallest of all.

Why use یکی after این instead of just saying این?

While این simply means this, adding یکی translates to this one. It highlights a specific item out of a set of physical choices, which is very natural in spoken Farsi when you are pointing out one specific thing from a group to call the tallest.

Could we use از to mean of all here?

You would use از if you were using the comparative form, such as از همه بلندتر است, meaning taller than all. However, because this sentence uses the superlative form بلندترین, the Ezafe construction is the correct grammatical way to link it directly to همه.

If we wanted to specify what this object is, like the tallest tree of all, where would the noun go?

The noun would go immediately after the superlative adjective and take the Ezafe itself to connect to the group. For example, you would say بلندترین درخت همه to mean the tallest tree of all.