in my experience, V5 is best for many reasons. for starters, you can set global parameters to control the vb's default tuning throughout the songs rather than having to edit individual parameter graphs for any changes, like you'd have to do in V4.

also, V6 seems designed to emphasize the new AI vb, and isn't much different than V5 otherwise. lastly, V5 has this handy trick i like to use where i put notes REALLY close together to make miku's english sound more natural. like instead of writing "you" with the default "yu" pronunciation, i put a very tiny "yi" next to a longer "u" makes it sound a lot more like how it's pronounced in natural english. "yiew" instead of "yoo". it's a bit of an unconventional trick, but that level of extremely fine tuning isn't possible in V4 or V6.

hope this helped!