Scrivener is the ducks guts, when it comes to writing tools. And as stated it isn't on iOS
yet, but will be by end of year. I bought it 1 month ago, for $15 in a Special, and as someone that hated writing at school, has found I am a writer sort of writing my life story lol but its a awesome App, and when it comes to iPad, WOW !!!
If you are looking for Minimalistic then I use Plain Text 2 on my iPad Mini. It has settings to set font, once and forget, you can change line height, and syncs via iCloud or Dropbox, has a good Folder structure, has a Extended Keyboard, (so you don't have to keep going into
other characters), supports Fleksy Keyboard and has word and character count. It also supports, plain txt, Markdown, .html and more.
If you are looking for something a little more robust and something where you need formatting, then there is as stated above Word, but nothing beats Pages from Apple, and with the integration thats coming with iOS 8 in the Fall, then Pages is your best bet. You will be able to start a document on your iPad, then when you are within range of your Mac, Pages will be ready to continue from you last keystroke you did on your iPad, and this will be seamless.
If you want a 3rd Party App, ill say 1 word . . . . .
NOTABILITY
EDIT: And OneNote from Microsoft is pretty good as well, and is well worth the look