Best Snowballers?

You want snowball? Skywrath is actually decent.

spectre can't snowball for ****.

FTFY

Spectre can jump on anybody with global range and with a maxed Desolate kills any hero in 4-5 hits. Because she's a hard carry people often forget that she can fight with literally zero items as soon as she hits level 6, and deal absurd damage at this point in the game.
She makes up for her garbage laning presence by being *really* good at killing heroes at any stage of the game.

She's not the "best" snowballer but she can snowball just as well as Phantom Assassin without question.

Nobody has mentioned Nyx Assassin?

BKB

Oh and what about Chaos Knight? That dude has some nice snowball capability with his illusions that deal FULL damage. Not to mention all he really need is a Soul Ring and Drum of Endurance to start ganking.

Once Night Stalker gets a few successful ganks, and picks up a few early items like Power Treads, Helm of the Dominator and Skull Basher he becomes pretty difficult to deal with.

I think that Slark can take this role pretty easily; everything about him is made to get kills, out level, out farm, and survive against almost any line-up, but against true snowball heroes (ones with permanent bonuses for kills, as in Legion Commander, Silencer, and Pudge), he eventually falls in place due to his awful stats.
I do agree that Storm Spirit snowballs hard, but the game of throws is unknown to Slark.
Also, I am surprised no one mentioned the Outworld Devourer. He can snowball so hard. Let him get ahead (which is really ez for him) and the two or three shot is real.