Bill,
There is no best chemical as all draught marks are not the same. In different circumstances I have found Stain pro, ultrapac renovate, filter pro and eco spotter to be the "best chemcial"
As important is agitation with a soft brush. This greatly improves the performance of the chemical. Extra time spent agitation will pay off in results.
Another factor which is important is heat. The hotter the rinse the better the result.
My favourite tool for draught marking is the cfr tool. It allows you to clean right up to the edge and won't overwet the carpet easily.
In a badly draught marked room I would test each chemical to see which is going to work best. Then spray and agitate a section of draught marking before rinsing through with a cfr handtool. Once all the edges are complete i would then clean the room as normal.
Hope this helps
Nigel