Best app for creating disk images?

[TribbleSmasher]

None of those formats are actually standards as they vary in size and layout.
IMGs for example can mean really everything.
Same goes for a disk. if it is AmigaDOS you can use TSGui(tracksaver) and name the image whatever you want but it is a raw copy of the tracks of a floppy disk.
If you use it on harddrive partitions it will work but if you name the image .iso will not magically turn the image into a disc.

Btw, you can get TSGui here or from Aminet.
https://thomas-rapp.homepage.t-online.de/download.html

[TribbleSmasher]

For floppies (ADOS) you can use TSGui, for hdds (probably flash drives anyway) you can use ImageWriter(Win) and for discs ImageBurn or AnyToIso.

To create disc ISOs on Amiga you can use any authoring software for CDs, like BurnIt, FryingPan, MakeCD.