Android - Which BusyBox is legitimate on Play Store?

Use either of these:

• BusyBox
• BusyBox Pro (paid)

Note this comment in the description:

Please note I did not write BusyBox! I wrote this installer and cross compiled BusyBox for Android. Please see the about menu option for more details or here: www.BusyBox.net

If you're installing a custom ROM, it might have BusyBox already (but usually not Pro).