What kind of stone makes the most porous fill? (french drain)
You are looking at 'void fraction'  Anything of mixed sizes has a smaller void fraction because the little guys snuggle into the spaces left by the big ones.  

If all the rocks passed through a 2" screen and not a 1.5" screen they are close to the same size.  Doesn't matter much if they are round or jagged.  

It also doesn't matter much what size they are, at least until you get down to sand size.  Sand runs about 30% void.  So does big rock.  But a mix of sand and big rock will end up with sand filling 70% of the voids in the big rock.  so you get 30% of 30% void = 9%.

You will lose some volume over the years from debris infiltrating in.  If you can come up with a scheme to trap debris at the surface, this is easy to fix.  E.g. Stop your fill 1 foot from the surface, put down filter fabric, and put a more rock on top of it.  Now, if it starts acting slow, take off the top foot, wash the gravel, put down new filter fabric, replace.