The power of the whole Infinity Gauntlet was needed to fulfil Thanos's master plan, but if the special effects are any indication, it was one stone in particular that did the job.
In a wide-ranging interview with Inverse, Infinity War VFX Supervisor Dan DeLeeuw discussed the design behind the infamous dissolve-to-dust special effect used to dramatise Thanos's victory at the end of Infinity War, saying that it was based particularly on the Power Stone, one of the six Infinity Stones Thanos had gathered by that point.
"The Power Stone was blipping them out of existence", DeLeeuw said. This decision, which has both narrative implications as well as design ones, was made after a lot of back and forth with regards to what effect would be best. What would each stone look like, erasing characters? Eventually, they settled on just one stone.
"We had concept art combining all those things and it was getting too busy", he said. "It became too complicated. It was stepping on what the actors were doing. So it became 'body turns to ash.' We peeled away all those layers and focused on that one [Power Stone], deciding how quickly it would consume someone, what pattern it would consume them".
Whether this means that only the Power Stone did the work of erasing half the universe, or if the other stones helped out (maybe the Space Stone augmenting its power across large distances, or the Reality Stone indexing all of reality or something), is up for lore nerds and fans to figure out. After all, he had to have collected all six for a reason, right?