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So what unit is each legion known for in the post heresy? I know world eaters are the berserker, death guard plague marines, etc. But what about the other 5 legions? How are they known for fighting in the post heresy and what units would their armies use?

Nightlords: Raptor units.
Iron warriors: Siege engines (as represented by use of cannon).

Not sure they have any one iconic unit. Black Legion seem to favour their sorcerers more than the other legions but you'd mostly find them in command positions. They're known for embracing possession though so perhaps possessed would fit the bill.
Iron Warriors probably use their slaves the most in combat so I'd pick them. And probably obliterators in terms of marines.
For word bearers I'd go with human cultists.
I'd give alpha legion chosen and finally Raptors for night lords.

Word bearers are cultists?

And traitor chaplains.
They go into chaos religion and summon hoards upon hoards of fanatics.
Possessed too.

Iron warriors, vindicators, havocs?

Night lords, raptors?

World Eaters: Khornate Beserker
Death Guard: Plague Marine
Emperor's Children: Noise Marines
Thousand Sons: Rubric Marines
Night Lords: Raptors
Iron Warriors: Siege Units
Word Bearers: Dark Apostle
Alpha Legion: Cultist
Black Legion: Doubt they have any, to be honest.

Would obits be an iron warriors unit?

Per Storm of Iron they appeared to be, created by technoviruses vs. warp magic. Other chapter could have similar though but leaning more on the warp magic babble vs. technobabble.

It is really difficult to say as the legions are so fractured.

All the Tsons became rubric marines or sorcerers

All the deathguard that turned to Chaos became plague marines

Most of the world eaters had butchers nails and followed Angron in his love for in your face assaults/close comabat

The Word Bearers are the most coherent legion, bound in their faith and love of possessed

Beyond that you really cant say.

Black Legion are fragmented and high varied. The only thing that might single them out is a lot of Chosen

You never can tell with Alpha Legion. They bring to the battle what you least expect.

Iron Warriors love siege warfare, but there isnt really much in the codex to represent this. The Daemon Engines, Oblits, Mutilators etc are more representative of the Dark Mechanicus than Iron Warriors.

The Emperors Children did not all fall to Slanesh. The process where Fulgrim did fall completely fractured and fragmented their legion though. You will not find a large Emperors children warband, but if you do it will be all devoties of slanesh.

Night lords specialize in terror tactics, not necessarily Raptors(although the OOP raptors certianly looked like Night Lords)

Word bearers: Possesed (gal vorbak) and Dark Apostles.
Black legion: Chosen (reavers) and Termis (justaerin)
Alpha legion: Cultist.
Iron.warriors: Oblitetators, Muitilators, Maulerfiends, Warpsmiths and Havocs. (Any daemon machine)
Night lords: Warp talons and rapaxes

Emperor children: noise marines
Death guard: plague marines
World eaters: berserkers
Thousand sons: rubric marines

I always thought chosen would fit more with alpha legion?

They do, but like I said it's really hard to choose one iconic unit for the undivided legions. Least the ones that are on the battle field and even harder for the table top units.

An alternative would be to go with dark apostles for word bearers, being their highest leaders and unique to them.
Defilers for Black Legion since Abaddon is the one that commisioned them.
Obliterators for Iron Warriors for being likely to be involved in their creation.
Chosen for alpha legion given how they operate so many individual cells and thus forcing the individuals to be rather proficient in many skills.
Raptors match night lords but it's not like every night lord is a raptor. They do tend to have more of them though.