To go through all the
IoM factions:
C:
SM- The downside of this is that the core of it is just more of the same Tactical/Assault/
Dev/Termy/Bikr/Vet squads that make up your current armies. The upside, though, is that you get all the cool toys that
DA and
BA don't have access to. Thunderfire Cannon, Stalker tanks, Troop Bikes (cheaper than Ravenwing and not available in Blood Angels), Centurions, Stormtalons and the glut of special characters that C:
SM have. Almost any army can use Tigurius or Loth for psychic goodness, and with
FW and the Codex, there's a ton of
SCs and
CTs to pick from, so you can plug a lot of holes.
IG- As said above, bodies and blasts, especially useful for
BA, but also for
DA. 85 points for 21 Fearless ObSec wounds that can re-roll hits and maybe wounds in
CC if need be (Conscripts with Priests) is great for chaff or objective sitters. Cheap AV14 in Russes can help provide some awesome firepower but also add some armour saturation to take the fire off any Rhinos/Raiders you're bringing. You also get some highly specialised units like Vendettas, Veteran Squads and Ogryn that can fill their given roles (anti-tank, AP2 spam and
CC respectively) relatively cheaply. You can also pull gimmicks like Priests in
SM squads or Azreal in a Guard blob for a 4++, but I'm personally not a fan of that sort of thing.
SW: One of the best, in that they have great options in almost every
FOC slot. Pretty cheap Termies, super-versatile
GH squads that excel in pods, dirt cheap psykers, ThunderCav, empty Drop Pods should you need them, and a beatstick
HQ to rival SmashFether. And for all those who say
DA and
SW shouldn't ally, it's actually really fun and fluffy to do so, as while they do have some enmity, they also have a grudging respect. It also gives loads of opportunity to play in character, with the
DA hanging back and tutting as the
SW go haring off to hack stuff up. Just be sure to fight a duel between their 2
HQs before the game begins to make sure honour is satisfied!
GK: I'd advise against these guys really, the last thing
BA especially need is even more expensive models to go with their overpriced stuff. While they bring awesome Termies, great psychic potential and some really nicely killy Dreadknights, a
GK/
BA alliance is just going to have way too few models on the table to do overly well. They might work with
DA as kind of a sub for Deathwing, if you take their special detachment that give T1
DS and only requires 1 Troop choice.
MT: Not worth it, as they are far too limited in scope and can be done just as well by using the
IG codex.
Imperial Knights: 370/375 points of plug-in ally goodness, throw in a Paladin or Errant into almost any army and it can work wonders.
Legion of the Damned: Limited, but if you need
DS Melta/plasma or a semi-decent tarpit for high-
AP CC weapons, they can be pretty good. I'd recommend them if you bring a lot of Pods.
Inqusition: Like the knights, something of a plug-in-and-play option. Inqusitors bring all sorts of fun and deadly stuff to the table, so whether you need psykers, warp charges, cheap objective sitters or
CC units, there's something there to help. Probably not something you could build a full detachment from, though.
