Well that and he just does everything else so well. He has a spammable W that can wave clear, do poke dmg, and snipe fleeing heroes. He's got an aoe snare for CC, oh and he's the only hero in the game that can restore mana for allies.
To top that off, he still gets all the other goodies like cleanse, shrink ray, scout drones, bolt of the storm and iceblock. No other healer in the game has even half the utility that malf does.
Where every other healer has their drawbacks. Malf has none cause his utility and options are just so insane.
edit: And I just got the Brightwing announcer from a Brightwing box, feels good man.
Yeah well put, it's the buff to twilight and his e being usable in a non-followup situation that do it in my book.
But yeah right now i think every single healer is strong and on top of "why take a melee assassin ill just take another warrior" it's "why take a ranged assassin ill just take another support"