Secretion in Gram negative bacteria
I think you cannot find a better answer then the following review:

http://www.nature.com/nrmicro/journal/v13/n6/full/nrmicro3456.html

It was published in 2015, and it gives a good overview about the secretion systems. Anyway, T6SS can also be virulent. Vibrio cholerae can use an effector of this SS to cross-link actin in the host cell and modify its morphology and its cytoskeleton behaviour. 
In general I think that there is not a general order of virulence, also because the T3SS was discovered long ago. Whereas, the T6SS just recently, so the amount of information about its function is still somehow limited. 

Hope this helps.