You should complement Retroarch with
Mednafen for GBA.
Due to frameskip*, it plays some games ''faster'' then VBA next, especially the ''3d'' ones or some like Zero Mission etc. For many others, RA is the better choice. Those 2 combined give a 99% playability of the GBA line up.
And before anyone says Mother 3, thankfully there is an inject wad for that
*This also happerns in some instances with SNES 4.2.9 vs SNES 9X next 1.5.4 in portions of games like Super Star Wars mode 7 stages, again, due to frameskip. RA is way more accurate of course.
Fba core. In general for ARCADE games you use fba core, for CPS1 games CPS1 core, same for CPS2 and neo geo.
Make sure to check the
awesome list compiled by groose to know which games work with which core.
Also when looking for roms, search for a
FBA romset so that you have correct roms.