Translation:This is the most expensive diamond.
Yes. Yes it does. 「一番」(いちばん) lit.: number one. And it's japanese equivalent of adding "most" or "-est" to adjectives. Like 「一番安い」(いちいちばんやすい) would be "cheapest".
Maybe 「一番目」(いちばんめ) would be better for "first", but what do I know.
Kono daiyamondo wa ichiban takai desu.
This diamond is the most expensive.
I think either can be correct, but that が sounds more natural because we're pointing to this specific diamond, and が is the particle we use to emphasize the subject. If you use は it's more like saying "speaking of this, it's the most expensive diamond".
Pretty much the same sentence but use 安い(やすい) instead of 高い : 一ばんやすいダイモンドをください。
I spelled the katakana for diamond wrong but don't mind that (^_^;)
Would "This diamond is the most expensive" be incorrect? Because they marked it as wrong.