The ones that make me look at a card in a completely different light
Its still probably mostly a sideboard card, but it has enough upside that Im willing to take it over some cards Id be more likely to maindeck
On the surface, milling ten cards in a turn for each of your opponents might not seem like much, but add in a few other unblockable creatures and maybe a few effects that let you steal your opponents high powered creatures to attack with, and suddenly everyone is milling tons of cards each turn
its like having Archmage Emeritus plus Tatyova, Benthic Druid on board
Ive used Solitude to stop combo creatures, break up lethal attacks, and remove commanders at the worst possible moment for their controller
With reanimation spells and a sacrifice outlet, you can also take more control of the ramp and land tutoring, keeping you on pace with the greediest ramp players