I did lot of homework about this issue too, my final choice was between WP (NOT Works Performance!), Öhlins and Wilbers. There were other smaller handmade ones too, mostly in the US/Canada, but didn't bother to risk as an european myself, it could get expensive.
Wilbers was the clear winner.
Better pricing, fully setuped according to your individual
needs and riding style (spring type, preload amount, rebound, dampening characteristics, etc), better maintenance schedule (every 30,000 miles or two years they make it as new again) and maintenance price, the guys at Wilbers were very helpful and informative in advice, choosing etc.
Make sure you buy the hydraulic preload adjuster! Really handy on the GS since it's awfully messy to adjust preload w/o it.
Those are my personal experiences anyway... Which doesn't mean everybody has to agree with it.
Many happy'n'reliable'n'unleaky-shock miles, Margus