What's a good exercise for the upper / inner most part of my quads? I have good definition and mass on my legs but that particular location is spongy.
I believe you're referring to the abductors which are best isolated by the abductor machine, but it puts you in a bad position if you're a male and IMO it's just a ****ty machine.
Any hip dominant exercise that involves abduction is your best bet like wide stance PL squats, box squats, olympic squats, and sumo deadlifts.
That's not part of your quads; it's a mix of your adductors and semimembranosus. This area is one of the last areas where fat will be lost.
M.Ed. Ex Phys