Best deal depends largely on where you live and how far you're willing to travel to find it. Swap meet here and Craigslist will net plenty of options.
Best year, stock vs. stock the '06 probably was best of the class, but here we are 7 years after the '04s were launched and you're just as likely to find a clean '04 that has had any potential issues resolved as you would find an '06 or '07 that is now old enough to start having problems (possibly). The reality is none of them are inherently bad.
Just about any used sled, especially mountain sleds, will need some time and effort put back into them. You just have to decide what your total budget is, how much of the work you can do or are willing to do yourself, and then plug a sled in that meets your criteria.
Sleds have evolved so well in the past 10-12 years that anyone just getting into the sport or upgrading from really old iron has tons of choices that are usually far more capable than the rider. Sure isn't like it was back when many of us were getting into the sport during the haydays of free air engines and leaf springs under the skis!