Voracious Greatshark

The counter isn't a cast trigger; it's an enters-the-battlefield trigger stapled to a flash creature, and that distinction is doing more work than it looks. A conventional counterspell answers the stack and leaves you with nothing on the board; this leaves a 5/4 behind whether or not there was ever a spell to hit. Cast it as an end-step ambush blocker and the counter clause simply whiffs, no harm done. Hold it as open interaction and it functions as a Mystic Snake with a bigger body and a narrower net: it stops artifact and creature spells, but not the burn or removal aimed at your face. That target restriction is the price for stacking countermagic onto a real threat, and it shapes when you want the card. Against a control mirror it's a dead counter that's still a fine flash beater; against a ramp or artifact deck it's both the answer and the clock. The Snake lineage is worth naming because the design has been iterated on for years: a body plus a countermagic ETB is one of blue's few ways to interact at instant speed without falling behind on tempo. This is the aggressive-blue tempo read of that idea, where the 5/4 matters as much as the counter, and either mode alone justifies leaving five mana untapped.