If you mean to use for creating your own scene's (IE if you're developing your own game) then koikatsu has about the lowest specs needed for anything 3d (that I know of anyway). MMD might work and there's a plugin for exporting koikatsu models to MMD but I haven't actually tried it so not sure.
If you just mean for playing
this post has a list of games posted on here that used koikatsu for the renders but can run even on really low spec computers.