What is the best software to clean/clear used iphone before resale/disposal?

iOS. No additional software required.

Click here and follow the instructions.

(127037)

You wouldn't use any software - you simply go to Settings>General>Reset>Erase All Content and Settings.

Here is an article that provides additional info:

Prepare your device to sell or give away

Cheers,

GB

and pls make sure you're signing off from iCloud on your device!