If you want it, nows the time
Off the top of my head, in Standard and Modern, it prevents some or all of the use of the following cards: Every flashback card (in particular) Ancient Grudge Desperate Ravings Dread Return Dream Twist Faithless Looting Forbidden Alchemy Mystical Teachings Past in Flames Think Twice Unburial Rites Snapcaster Mage Narcomoeba Reanimate Exhume Every undying creature Every persist creature Gravecrawler Ichorid Ashen Ghoul Proclamation of Rebirth Polymorph Probably quite a few others 1) Drogskol Captain Hexproof is a very non-interactive creature mechanic, making it one of the more powerful mechanics to have been printed in recent years
The first card I jam is Basilisk Collar, so that Snapdaxs mutate trigger can kill anything, no matter how large
Whats really cool though, is that you can tap them to sacrifice X clues and, Target opponent reveals the top X cards of their library
Creatures like Feral Hydra can put +1/+1 counters on themselves
14 Hulking Metamorph Being able to clone a creature you control is a little limited in scope which is why Hulking Metamorph isnt higher up on this list