Here is a list of the file formats that I use within emulators, I prefer compressed files when possible as it saves on disk space.

Generation	System	Best File Format
Home Computer	ZX Spectrum	7z
	Commodore 64	7z
	Amstrad CPC	7z
	Amiga 500	LHA
Portable	Game Boy	7z
	Game Boy Advance	7z
	Nintendo DS	7z
	Sony PSP	CSO but don’t always work
3rd	Atari 7800	7z
	Sega Master System	7z
	NES	7z
4th	Sega Megadrive	7z
	SNES	7z
5th	Sega Saturn	CHD
	Playstation	CHD (or PBP)
	Nintendo 64	None are reliable
6th	Sega Dreamcast	CHD
	Nintendo GameCube	RVZ
	Sony PS2	CHD
	Microsoft Xbox	SquashFS
7th	Nintendo Wii	RVZ

7z

7z is a file format created via the “7zip” utility. It’s got good compression levels and most of the emulators for the older systems can read this format. Normal “zip” files can usually be used too.

LHA

LHA is an older compression format and most Amiga emulators can make use of this.

CSO

CSO is a compressed ISO, mainly used for PSP. The “maxcso” utility is usually used to create these files.

CHD

Compressed Hunks of Data. This was originally developed for MAME, the “chdman” utility creates these archives.

PBP

PBP files are mainly used for Playstation 1 games, and this file can contain multiple disc images. CHD is better but needs separate files so isn’t quite as convenient.

RVZ

RVZ is a special format used by the “Dolphin” emulator for Gamecube and Wii games. The Dolphin GUI is used to compress files.

SquashFS

This format is used for Xbox, PS3 and Wii U games. The utility transparently compresses games via the “mksquashfs” utility.

Wrap Up

Hopefully this might help somebody looking for the best compression formats to use in games emulators!