What's the best allied army?

[YorkNecromancer]

Simple enough:

Which is the best army combo?

Now, 'best' here is a nebulous term. If you like gameplay, then it's referring to synergies between separate codicies that allow them to become greater than the sum of their parts. If you like fluff, then it means something different, referring instead to an intriguing story idea combo. If you like both, then it's not merely a synergistic powerhouse on the table that somehow avoids being brokenly imbalanced, but it's also a great story hook to Forge The Narrative with.

Bottom line, 'best' is literally down to you, so be sure to explain what your definition of 'best' is before you make any comments.

And for those who want to complain that they hate allies, that there are no 'good' combos and they just want them to go away because they're overpowered/unfluffy/insert personal bête noire here? Please don't do that; this is not a thread for your complaints.

Mine?

Kantor + Scouts + Sternguard in various transports = Deathwatch + (either)Grey Knight Terminators (and/or) multiple MSU Sisters of Battle with Immolators + Inquisiorial Warbands for flavour = Full-on Inquisitorial Conclave that can seize ground and hold its own with the best of them.

Other suggestions?

[Mr Mystery]

Astra Militarum, Skitarii and Cult Mechanicus.

Just looks ace on the board. Broadly similar scale, complimentary aesthetics.

[Ruadhan2300]

I suggest Cultmech, Skitarii and Tempestus Scions.

It gives you a substantial force of heavy infantry with some nasty monstrous creatures and walkers. Supported by armoured cavalry and long range lascannon firesupport from Vendetta gunships.

Also their aesthetic details are practically identical, so in the same paintjob they might as well be the same army.
I have some Scions painted in Admech colours with mechanicum-esque capes that originally served me as Skitarii until the official kits came out.
They make great Skitarii heavy infantry to complement the Rangers and Vanguard as lighter troops and the Kataphrons as heavy weapons units.

[Charon]

Depends on the setting, restrictions and game mode.
In terms of raw power the "best" ally to every single army (yes, that includes tyranids) seems to be the imperial knight.

[Jamie Garrett]

AD Mech, Skitarii and my Imperial Fists. The robot conclave defending my Marine tanks at the rear plus lots of rhinos with tact squads in as mobile infantry works really well (followed by lots of arc rifles and Grav cannons)

[Mr Mystery]

Depends on the setting, restrictions and game mode.
In terms of raw power the "best" ally to every single army (yes, that includes tyranids) seems to be the imperial knight.

Astra Militarum, Skitarii, Cult Mechanicus and Imperial Knights because dakka

Cheers for the reminder

[Defenestratus]

Grey Knights.

[Kirsten]

Iron Hands and Mechanicus clearly.

and of course Necrons and Blood Angels...

[Deacon Ix]

Necrons and Blood Angels...

This

But then I like Chaos and Guard

Another one I quite like which I have played as a co-op - "The Angels" - Blood and Dark

[Kirsten]

chaos and guard are awesome