I'm a silver Illaoi main who regularly SHITS on teams.
When I have a good game people think I'm smurfing.
That being said, I've tried a lot of new runes, and here is my personal opinions.
Grasp: Decent, but underwhelming.
Klepto: Fun, but even more underwhelming.
Comet: Best damage early to mid game.
Phase Rush: have not tried... tempted to try.
Now what you all came for!
Weak as shit early game, but just wait.
at about 25 min, it will catch up in damage to other runes,
IF the game goes past 40 min... wow
It starts doing like 1k damage per fight, and I have not even done a super long game.
Late game you basically (E + Ult), and start getting kills. Each one resets Dark Harvest.
It takes the 40 minute Kraken to whole new levels.