Furyou Miko wrote:
Best Guard ally?

Elysians. Dedicated Transport valkyries free up your allied fast attack slot for Vultures, and you can put ground scanners and MotFleet in for reserves manipulation.

Paradigm wrote

What with Space Marines having just received a new codex, I though it would be worth a look at why, when and how to use them as allies to an Imperial Guard army:

For marine allies with guard, what you need to do is either A) bring them to counter a specific weakness in your lists or B) use them to bring more of what you already have. Option A is probably the best, as it adds versatility to the list, but B has its merits, and can use some good combos like a Drop Pods assault backed up by stormtroopers for even more backfield killy goodness, or a mechanised list with a couple of rhinos and a predator or vindicator chucked in to add to armour saturation.

On the whole, though, you should be looking at countering the 3 main weaknesses of the IG codex on its own: Lack or durability, lack of mobility, and lack of Close Combat prowess.

Lack of Durability: it is an unfortunate reality that guardsmen, particularly on foot, have massive problems with staying alive in the later parts of the game, and this is where the Space Marine codex can help out. Taking 20-30 marines, some/most of which are scoring can boost the survivability of the army as a whole, and also because the enemy will have to divert considerably more firepower to remove them than some Infantry Squads, even in cover. This gives you better objective-holding ability, and also helps in missions where kill points are in play. A good allied contingent here would be:

Chapter Tactics: Iron Hands
Librarian, terminator Armour, Storm Shield
2x 10 man Tactical squads (add weapons as appropriate)
TH/SS termies.

Gives you a good core of reliable troops, a pretty tough if expensive Close Combat unit, and has great staying power.

Lack of mobility: Drop pods seem like the obvious answer here, but consider for a moment the humble Rhino. A DP gets you to the enemy and then does nothing, a Rhino can keep you moving (and therefore reacting and capturing) until the end of the game if you are careful with it. Combine this with the Raven Guard Chapter Tactics, and you can have the enemy board edge in rapid-fire range by turn one, or outlfank your allies for a nasty side attack. Sample list:
Chapter Tactics: Raven Guard
Captain, Jump Pack, weapon of choice
2x 10-man Tactical squads, rhinos
Assault Marines or Vanguard Veterans for some mobile Close Combat if you want. If not, swap the captain for a librarian and shove him in a platoon for 'and They Shall Know No Fear' on a combined squad.

Lack of Close Combat ability: This is something that is less essential, and probably better covered by the Space Wolves or Dark Angels codex (or chaos if you're willing to go non-Imperium), but Codex: Space Marines can still hold its own. in this case you want to keep troops to a minimum, maybe even just a scout squad, and then put all the points you want in Elites and Fast Attack, getting ASM and Vanguard/Terminators (if the latter, get a Land Raider Dedicated Transport) and maybe even a Close Combat tooled Command Squad, on bikes to keep up with the mounted and Jump Pack elements. Good list:
Captain, bike
Command, bikes, Close Combat kit
Thunder Hammer/Storm Shield termies, Land Raider
scouts
Assault Marines, full squad, Jump Packs