Luffy , Roronoa Zoro , Shanks , and Portgas D
Improvise offsets the amount of mana you need by letting you tap artifacts you dont need untapped like Void Mirror and Damping Sphere, making this super well-rounded and insanely strong
And frankly, the only reason this hits back at #10 these days instead of a lot higher, is that the most commonly played multiplayer format, Commander, starts with 40 life instead of 20, so its not as painful there
This is synergistic with the "flashback" and "jump-start" mechanics found in recent Magic sets (see "Preview: 'Magic: The Gathering - Ravnica Allegiance'")
It's easy to cast along with its other burn partners and having multiple copies of essentially the same effect leads to all of your burn spells doing a ton of damage