Try Documents by Readdle. Despite the name, it is a very good file manager app with FTP and SFTP support. And it is free
A good paid app is iFiles 2. Also has SFTP and FTP support, and you can view files directly from the share without the need for downloading first. Tip: every now and then it goes on sale.
Another one might be Transmit. I haven't used this app myself but the reviews are very good.