General

@natty2: Phantom it is.
Cuz DrK has cd
DB is a hyper with cd
And hayato's meh

[quote=wspuma]You have to be able to move
A mobbing move, no arrows/bullets[/quote]

Phantom - Mille Aiguilles
Dark Knight - La Mancha Spear

and you may want to note
Dual Blade - Asura's Anger
Hayato - Sanrenzan,
as they both allow you to move while attacking.

You have to be able to move
A mobbing move, no arrows/bullets

I/L Mage - Frost Breath
Phantom - Tempest (cooldown) / Mille Aiguilles
Beast Tamer - Baby Bombers
Kanna - Vanquisher's Charm / Shikigami Haunting / Demon's Fury (Holding the attack button)
Dark Knight - La Mancha Spear
Demon Avenger - Vitality Veil
Demon Slayer - Grim Scythe / Soul Eater / Carrion Breath
Bow Master - Arrow Blaster / Hurricane
Mercedes - Ishtar's Ring
Wild Hunter - Wild Arrow Blast
Wind Archer - Song of Heaven
Xenon - Beam Dance (Obviously)
Angelic Buster - Soul Resonance (cooldown)
Corsair - Rapid Fire
Jett - Rapid Fire
Mechanic - Mech Siege Mode (Holding the attack button)
Thunder Breaker - Typhoon

Not really holding key down, but I think it's worth noticing:
Kaiser (Gigas Wave kinda)
Dual Blade - Asura's Anger
Hayato - Sanrenzan

Archers: BMs WHs Mercs WAs
Pirates: Mechs Sairs Jett
Mages: Lumi (spectral light if that counts) BT Bear mode Kanna
Warriors: Drks DS
Dont think theres any for thieves.
EDIT: just joking; phantom

phantom
wildhunter
sair
bm
merc
wa

Sairs, Phantom, Bowmasters, Wild Hunters, Battle Mages

if by keydown you mean blade dance, then:
phantoms, bowmasters, jetts, drks, wind archers and probably some others i'm forgetting

Beast tamers Bear, I think Dark Knight and then Demon Slayer. That's all I can think of atm.