This is actually a pretty quick answer. If you're looking for the flat out safest area it's southern France on either side. Just about zero offensive operations, in any definition of the word major, went south of Verdun. The lines would get further apart and the artillery use would go down as well. Both sides would have pretty good trenches there as a result

Now if we're talking about major combat areas you'd want to be in the German trenches in terms of trench quality. Noting the circumstances the Germans had the luxury of retreating to the most defensible position in France and Belgium, thus having the high ground near universally. As in unlike the Allied trenches they didn't have perpetual water run off flow into their trenches.

Further the Allies, for various reasons, had to maintain an offensive tempo in the duration of trench warfare (1915-16). So their trenches were just not as suited for defense as the Germans were.

However once we start hitting 1917 that may change for you. The Germans would abandon static defenses as would the Allies mostly. Further the Allies would be dining on eggs and liquor and even steak every night while the Germans were scrambling for old bread and broth. The blockade was really starting to hit them.