Why not just look it up in the store and see what's suitable for you? Everyone has something that they like over other things. Whether any of them give you everything is questionable.
I personally just use the standard email on WP. I don't need the fancy features of a Gmail app myself.
I personally like the built in email from MSFT. All the other ones I have used are basically web version re-skinned. Please note Google does not play well with MSFT and do if you are used to the full featured Gmail on Android or the browser you will be disappointed.
MetroMail has some adaptations for GMail to give you an archive button and show your replies inline in the conversation view. You can also choose which of your email aliases to send from, which is handy.