I've owned all of the above and I think the HP Practical is the best I've had. Still has a lousy trigger but is tactically set up much better (sights, safety, etc) I'd still reccomend having it worked on, however. But if you need it to go right now, get the newest one.
I had a 70's manufacture FN (and many years later sold it in a fit of stupidity).
The new MK III's are excellent pistols - but if the 80's pistols are without that accursed firing pin lock I would go with that one .... or get the '78 and have a competent gunsmith add a useful safety and sights.