For example, the '67 ticket has a couple cars which have a value of 20 million each. Is there any particular year which yields the best / most desirable cars?
I agree, most of the more expensive cars come from these tickets. You could just go to http://mygranturismo.net click cars and sort by year to see for yourself.
Depends on what you are looking for obviously. 2010 and 2009 tickets have a lot of expensive cars. Also, a level 17 ticket is pretty much a no lose situation, especially if you like the JGTC cars.
2000,2001 have a lot of collector's items also!They include a lot of cars though so you have to be patient and try it a lot of times probably before you get them.
I think they start to get good during the late 60s and increase in quality from their. They are best from '89 to '92 and from '98 to '00 IMO, although there are certainly good cars available in other years.