Think about the Habsburgs' weaknesses, and try to pick a civ that compensates for them. Also think about the weaknesses of other civs, and how the Habsburgs could mitigate the weaknesses of their teammate.
I'm not really an expert at them, but in my experience, the Habsburgs' main weaknesses are the high cost of their units, which means that they can be relatively easily overwhelmed by large numbers of fast melee units in the early game before they get large numbers of units themselves. They are able to harrass, but it's risky, and they can't easily win with a rush.
A great strength of the Habsburgs is their lategame defences - they can build large numbers of very powerful forts, so would be useful to couple with a civ that is weak in defence.
Taking strengths and weaknesses into account, it may be a good idea to try them with civs like the Tupi, Zulus, or Paraguay, which all have fast and cheap infantry which can act as a meatshield, and can rush effectively.
If you're expecting your opponent not to have an army in the early game Habsburg weaknesses might not be so important. It would make rushing more viable, so those civs I mentioned might still be good, but you could also go for another civ with a strong lategame, like the USA or Germany.