Stands, by far. Hamon was a cool power in the variety of ways it could be utilised, but only served a purpose when fighting vampires or pillar men. Stands, however, had a variety of abilities that made power scaling far less linear and battles work more as a series of mind games rather than the typical beatdowns part 1 was or the limited methods of utilisation for strategies in part 2, making it so no one Stand encounter was the same as the other.
The only thing I somewhat prefer about Hamon was that it took years of training and discipline to master, though considering that was already being lessened in part 2 and Stands still require the prerequisite of having strong willpower, that barely holds up.