Ion rockets are probably the best bet...
If you just want efficiency- if you need high G thrust they are useless- the gas ion drives use is Xenon, which can be frozen by LN2 (I nearly destroyed a few thousand dollars of equipment because I neglected that fact as a student)
At my last job I actually got to see LOTS of xenon ice- it was kinda cool (pun intended)
We probably made more of the stuff than had ever been made on Earth in 4 billion years...