XATA is telling me that my Fitness Signature is stale and that I should attempt a breakthrough. I’ve had a look at both the “Standard” and “Coach” Breakthrough workouts and there are quite a few of them. Is there any recommendation as to which one is the best one to do?
Cheers
Different strokes for different folks. 
Any of them should do the job.
If I search by “breakthrough” I see six possibilities currently (4 standard, 2 coach).
Try them all one by one over the months ahead and see which one(s) you like best.
If a BT workout doesn’t challenge you like it should, ride it again in resistance or slope mode and go all out during the work intervals.