for NES my CRT likes
-240p screen format
-640x480 render resolution for the screen format
-717x552 (5 x -40 offset) retroarch scaling resolution
-bilinear filtering set to off

For SNES I use a 702x496 retroarch custom scaling resolution with a 12 x -8 offset and bilinear filtering on. Genesis looks pretty good with bilinear filtering on and standard 4:3 resolution