I can't really tell you the best app to scan with (there are so MANY at the AppStore!), but I can tell you the one I use and can highly recommend. It's an app called Scanner Pro (link:
https://itunes.apple.com/us/app/scanner-pro-by-readdle/id333710667?mt=8). Right now, it's on sale at $2.99 (USD), down from $6.99. It has the capability to link to Dropbox and Evernote, plus you can email a scan. I use it to scan my debit and credit card receipts, which I send to Dropbox for safekeeping.
While I was looking for the link to the above, I also saw this by the same developer. It's called "Scanner Mini" and it's free (link:
https://itunes.apple.com/us/app/scanner-mini-free-document/id581365763?mt=8). Maybe this might work (also or in addition to)?
I'm sure others will be along to give their input. Happy researching.
Marilyn