Learn Turkish: Hangi elbise en pahalı? - Which dress is the most expensive?

En means "most". To say "the most...", you simply put en right before the adjective. So, pahalı is "expensive", and en pahalı is "the most expensive". Notice that Turkish doesn't need a separate word for "the" here!

If I wanted to ask "Which dress is more expensive?", how would this sentence change?

You would just swap en (most) for daha (more). The sentence would become Hangi elbise daha pahalı? (Which dress is more expensive?).

Where is the word for "is" in this sentence?

In simple present tense sentences, Turkish doesn't need a separate word for "is" when talking about an object. It is simply understood. So, Hangi elbise en pahalı? literally means "Which dress most expensive?" but translates perfectly to "Which dress is the most expensive?".

Does elbise need to be plural if we are choosing from a large store of dresses?

No. In Turkish, when you use a question word like Hangi (Which) to single out an item, the noun that follows stays singular. You just say Hangi elbise (Which dress), even if there are hundreds of them.