The following is my personal view and relatively limited to the Limestone Coast.
I find that there are similarities between the 12/13 and the 06/08 vintages (for a recent example)
08 and 12 are highly hyped, and a lot of the wines are bigger (Alcohol and fruit wise) than the 06 and 13.
However....I feel that these bigger riper styles (for a lack of better descriptors) will not have the legs cellar wise that the 06 has already shown, and the 13 should do so.
A lot of 08's (and some 12's) I have seen lately, tend to be a bit flabby and do not exude that leaner acid and lineal structure that 06 (and I believe 13) have, which in my eyes denotes that they will not survive longevity well. Bear in mind that this view is aimed primarily at Cabernet and Coonawarra in particular.
The 09 vintage also fits in with 06 and 13, albeit with more greener tannins in a lot of wines, but the structure is still there.
A lot of the problem is that with vintage reports being compiled by marketing and Vignerons associations, every vintage is the best and any facet that can be harnessed to reflect this is deemed as good. This also plays into the hands of people and companies that want to push ready to drink now wines from an area like Coonawarra, which as we know tend to need more cellar time to hit their straps. These market forces have caused a shift in a lot of winemaking practices, which have resulted in the bigger fruity higher alcohol wines at lower price points in particular. The more traditional styles that will age long term are primarily now at the higher end of the scale. I know this is a bit of a tangent, but I feel it is relative to good/bad vintage thing.
There are vineyards that will deliver year in year out, even in average years like 07 and 11 (for Coonawarra).
The better vintages from this region to me are the slightly leaner cooler years (Talking predominately January to April period) where the acidity and in turn structure becomes the key to the wine's character, and the classic subtle Cabernet flavours become apparent rather than over ripe fruit and alcohol.
So I guess in summary (after that ramble) don't believe the hype in some area's until things settle down. Not very helpful if you are looking at buying wine now however.
And I can guarantee you that almost anyone you ask, will tell you the best vintage to buy is the one they have on offer for sale at that particular point in time.
So in relation to Coonawarra, it depends on what your preferred style from the region is, as to which Vintage you buy.
Personally I would go with 13 (based on what I have seen and my preference for laying it down for years).
Or we could break the regions vintages down into sub regions and how they performed, which is another can of worms.......