Sure, piCore is a great OS for the RPi. If configured accordingly, one can turn it off just like an old school console. A friends RetroPie installation stopped working after SD card failure - This might not have been related to the OS directly, but still piCore might be a good choice to prevent unneeded disk access. OK, booting takes slightly longer than most consoles I guess. My current installation on the RPi3 boots to the desktop in about 16 seconds. Still very good for occasional gaming. I haven't tried to build Emulation Station on the Pi3 yet. I could imagine that it's somewhat Debian bound, though. So maybe not that much of a nice adventure. I had lots of trouble with Mupen64plus already. Still haven't got it to work.
What I have working:
PiSNES (Super Nintendo emulator)
PCSX-ReARMed (Playstation emulator)
Quake III Arena
Looking on my x86 games, these might make sense on the Pi as well:
Still looking for a good Tetris clone.