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Possessing as I do an SoB army, I've given a good deal of thought to allies. Here is a core SoB list and thoughts about allies. Feel free to add your thoughts.

Core SoB:
Two squads of ten Battle Sisters in Rhinos. I generally use heavy flamer, melta gun and combi-melta.
Two or three squads of Dominions - I use flamers and a combi-flamer - in an Immolator with twin-linked multi-melta. If, only two, then one squad of Seraphim.
Three Exorcists

The HQ might vary; without allies, these days I would play Celestine along with a large squad of Seraphim with two dual hand flamers. Uriah might make an appearance in the presence of some allies.

What I want from allies for my Sisters is help in the troop department, and additional ranged firepower. Close-combat ability is also a thought.

Imperial Guard:
Pros: Battle Brothers, cheap troops, multiple troops per force-organization chart-slot, good flyers, plasma out the Wazoo, decent heavy support options.
Cons: Squishy troops or expensive troops (big blobs aren't cheap), mediocre psyker - at best.

Uriah and a blob just screams to be tried, and I shall at some point. Plasma Veterans appeal as does the Vendetta. I do also love my Executioner.

Space Marines:
Pros: Multiple troops per slot also possible (true of the other Marine types as well), cheap troop option available. Decent psyker (Null Zone at least is good), lots of decent options all aroundincluding Rifleman Dreads.
Cons: Not Battle Brothers

Blood Angels:
Pros: An excuse to bring out the old, heavy-flamer Immolators in a counts-as role? fast vehicles. Moderately cheap troops (five-man assault squads are quite playable), FnP
Cons: Not Battle Brothers

Dark Angels:
Pros: Wait a minute, I'm thinking. Troop terminators might appeal to some, but they're expensive for my taste. Five-man tactical squads are playable. Expect DA to become more appealing soon.
Cons: Not battle Brothers

Grey Knights:
Pros: GK stuff is better than everything else for slightly higher costs. Cheap troops are available and good troops available. Good psykers, Psybolt stuff rules. The possibility to rehabilitate all those Death-Cult Assassins and Crusaders is interesting.
Cons: Not Battle Brothers

Space Wolves:
Pros: Space Wolf stuff is better than everything else except GK for about the same, or often, slightly lower costs. Good psychic defense, good psychic powers, five-man Grey Hunter squads are perfectly playable.
Cons: Not Battle Brothers.

While the IG may seem the obvious choice - and they're definitely worth a try - I'm really leaning towards the Puppies, just because of the Rune Priest. GK are probably the third choice with Space Marines an honorable mention.

Isn't this a tactics discussion? One which has already been had a few times. Look under tactics in this forum.

IG are the one army which might thank you for allying with them.
They are the ally of choice.