Different people are running varying sets. Popular ones are 6.72, 7.52, and the new 1.x.x versions are starting to get a bit of use.
In all honesty I think the 7.xx versions are probably the safest bet as they support DX8 features and seem to have good speed. The 1.x.x versions are a bit too new and people have reported a few bugs in them.
Of course, nVidia would only say to use the latest official detonators (ie. 6.50 from their website), or your graphics card makers (who will probably have a set based on a newish detonator version).