Battle.net, with the feature to run partially-downloaded games and stream missing assets in on demand.

This one is actually a game changer if you play WoW for a bit every few years lol

Game’s like 100 gigs by now, but you can get started after just a few gigabytes downloaded. Like 5 minutes on even my mediocre 150 mbps downlink.

In contrast, it used to take me several hours to get into it on a slower connection before this feature.