Without getting into the philosophical paradoxes inherent in asking about "the most expensive free trip", I'm pretty sure the longest duration & distance that you can pull off with an Opal card in a single day without backtracking is Goulburn, NSW to Scone, NSW by train:
This clocks in at a cool 513.54 km per the official fare/distance calculator and would take you just short of 8 hours (7h 57min, to be precise) with a single connection at Central.
Farewise, though, any distance beyond 65 km is charged the same, so it's just $8.30 for the whole trip. Since Opal cards max out at $15/day, it's not possible to get more financial benefit than this out of any itinerary. Because you're charged every time you transfer, it's quite easy to hit the $15 mark, even on much shorter journeys (say, Circular Quay-Manly return at $14.36 plus any trip anywhere).