Sneak Attack is second best in this category as the upgrades can be a bit more expensive, but its a joy to play and a fantastic stocking stuffer
You can summon Sabrina from your Hand when you feel that the opponents Active spot is threatening, instantly giving you an advantage as you eliminate their potentially winning spot
Self-Bounce Pricey Artifacts Rakalite / Cyclopean Snare / Razor Boomerang This Trio of Torture gives people the ability to do something decent at best, and then to bounce it back to your hand as part of its effect, thereby requiring to recast it over and over and over and over and over and over and over and over again
Green Goblin, Nemesis, a four mana Legendary Goblin Human Villain from Marvels Spider-Man Eternal, has gone from a near bulk rare to a genuine chase upgrade
I think she only has them at conventions, but if your paths cross you must check them out
Ive used Solitude to stop combo creatures, break up lethal attacks, and remove commanders at the worst possible moment for their controller