Force of Will Force of Will is the gold standard for free countermagic (and perhaps also for funny proxies!), and it's Magic's best free spell overall
It does not, for example, change when you may activate abilities that can be activated "only as a sorcery." The last ability cares only about the amount of mana that was actually spent to cast the spell, regardless of its mana cost
Players sometimes justify its inclusion through logic such as "If you mill six cards from your own deck, and the only flashback card that you turn over is a Think Twice, you're still going to be even on cards." But in green-blue-black mill-yourself decks that are actively looking to get as much of their library as possible (and in some cases all of it) into their graveyard, finding ways to continue to mill yourself is absolutely crucial
While it can also benefit your opponent in two-player formats, it becomes highly exploitable when your deck is built around massive creatures
Claim Jumper has the potential to get two Plains out of your deck