What is the safest technique for returning very aggressive serves?
A deep slice cross-court is both easy on your body and effective. With no knowledge of your skill level, using more of a "blocking" technique with your backhand may be easier to implement if you don't have the strength/quickness to pull off the Federer-style backhand slice (not many do!). Many aggressive servers begin a point with the goal of getting to the net as quickly as possible, but returning the ball deep while reducing the pace will set your opponent back on their haunches and take away one of their strengths.
You won't hit many winners with it, but it'll save your arm and help you cope with heavy hitters.
Naturally the answer depends on what you mean by aggressive.
The physically safest way to return most serves is by slicing them back with good timing. This puts less strain on tendons and joint than other strokes. As mentioned in the comments, Federer is a prime example of how to seemingly dissipate pace with well timed slices.
Aggression involving bursts of rage and violence from the opponent is best countered with a well placed smash right in the other guy's hea-cough.