Who makes the best allies for Knights?

[Ivarr]

Assuming a primary knight detachment; 3 knights.

Ok, so I initially was thinking space marines of some kind, but I am no longer convinced that they are the best choice. AM (IG) might do a better job; more bodies on the ground, armor options, fliers, cheap techpriests? Also considering AS (sisters) just because they are so very cool. I do not want to step outside of the Imperium for fluff reasons, but I would really just like every ones input.