I'd suggest Blood Angels. They are very fast, good up close and are just as resiliant as normal Marines, if doubled up with Sanguinary preists, they become even better! Plus they are 'Battle Brothers' with Guard, meaning you get all those added bonus's!
If Blood Angels arn't for you, I'd say either look at Space Wolves or Grey Knights.... Both are great allies for Guard! The possibilities are endless, but theres my two cents! Good Luck whatever you choose!
My Guard Army only consists of a Steel Legion Officer and a Blood Angels Captain right now! Oh... And a Commissar..... Possibly a Russ if I can get a Battle Cannon for the Turret..... But anyway, Sorry for Hi-Jacking this thread...
My personal opinion is that Blood Angels, Dark Angels, and Space Wolves make the best Allies for Guard. They're all Bros, and they all bring Divination, which is an amazingly useful power set for Guard.
Space Wolves are my favorite of the three, because they have the best Psychic Defense and because Grey Hunter packs are excellent at advancing to grab Objectives when they have Guard Fire Support.
Blood Angels with Assault Squads as their Troops would probably be the next best at filling in those gaps. More mobile and aggressive, but less flexible, and their Psychic Defense is more limited.
Dark Angels Troops tend to double down on the Guard shooty approach, rather than complementing it, but they have the cheapest Divination Psykers. Going to have to reach into Elites or Fast Attack to get (Counter) Assault Units here, tho, rather than being able to fill that through the Troops you're required to take.
Yeah, I gave a brief rundown above about why SW pair well with Guard, but I'd generalize beyond that, and say that they're one of the best Ally combinations in the Game if you want an all-rounder force. The two Armies just cover each other's weaknesses so well.