The knights I allied in were House Hawkshroud. This was for two main reasons. Firstly, their yellow primary colour scheme contrasts well with the templar scheme, while retaining the black and white we all know and love. Secondly, they are more likely to be "oathsworn" to a crusade or army than individual knights from other houses. I also appreciate their insistence of answering all calls for aid, regardless of the cost.
If you want to add a detachment of multiple knights, lore wise it shouldn't matter what knight house or forge world they are from. Oathsworn knights are rarer because they do not leave the armies they support during a campaign and often bear heraldry of the armies they support on their pauldrons. There are also freeblades, survivors of destroyed knight houses which operate more like mercenaries. These are great choices if you want one solitary knight, as they represent themselves, and not any army or knight house.