Varus
With options to play both lethality and on-hit builds, Varus has dominated the entirety of the summer season in competitive League of Legends. The alternatives have disappeared little by little, making this champion one of the most comfortable due to his high damage output and the safety of being able to hit from an almost insurmountable range.
Varus enjoyed a competitive presence of 74%, a figure that has remained remarkably stable throughout the last six patches.
With no changes planned, Varus seems one of those inevitable champions to kick off Worlds, with only the tournament-specific meta possibly changing that.
Other candidates
The ADC role doesn't have a great deal of variety right now, but there are at least a few viable alternatives.