I know people are sometimes morally opposed to the idea of "rental" software (I was), but I'm here to tell you something: if all you need is Photoshop, you are FLAT OUT CRAZY not to be using the subscription-based CC version. Here's why:
1. It's $10 a month. That's $120 a year, with upgrades included. You are about to pay something in the neighbourhood of $600-800 (maybe $100 less if you're lucky) for a program that is no longer supported and will never get another update ever. How many YEARS of Photoshop CC "rental" does that cover? If they were still on the CS system, you might well have upgraded to CS7 or CS8 when your CS6 would no longer work with (future OS X version). So that's another upgrade fee sometime in the next two or three years.
2. For that $10 a month, you also get Lightroom. Now, I'm not a huge fan of Lightroom, but its not bad. You also get 20GB of cloud portfolio storage for your work with Behance, and a few other minor goodies.
3. Photoshop CC 2014 was a HUGE UPGRADE (essentially, CS7 if they'd carried on that way) and IMO much better than CS6.
4. The thing about subscription payments that I like is this: I can, if I want, unsubscribe. Say I don't find myself using PS CC much, using more Pixelmator or something. Just stop paying. No harm, no foul, you can pick up the subscription again when (or if) you ever feel like doing so. If you find that your work is largely seasonal, you can rent on a slightly higher month-to-month need.
5. Still investing in lots of plug-ins? At the moment, they'll all still work fine with CS6. But that won't last much longer -- and all that money you've spent on them will be gone in a couple of years unless you upgrade. Not to mention that eventually CS6 will break under some future OS X upgrade. Probably not real soon, but eventually.
So for people who use PS all the time and upgrade fairly regularly (I used to upgrade every other version), the program is now dramatically cheaper than it used to be. For people who only need PS from time to time (but when they need it, they need it!), the program has gotten super-duper-duper cheap, since they only pay for when the use it.
If you can find CS6 for super cheap, then maybe its worth it to buy still. But you're going to kick yourself if Adobe ever raises the price on this $10 a month deal, because one day you *are* going to upgrade to it. At the price of maybe 1.5 lattes from Starbucks a month, I think this thing's a no-brainer, particularly if you actually use it to help you earn money.
And that is great if you only need Photoshop. chas_m, I was like you. I started out with Premiere 5 and over the years worked up to CS6. I purchased most the upgrades along the way which, as you know, varied it price.
In my case, I need more than one app. I have to have Premiere Pro, Indesign, Photoshop, and Illustrator. I could forgo After Effects (sigh) and the others in the suite if I had to. But it is irrelevant because with all I have listed I would have to pay for the whole suite anyway. Right now I can get a discount for one year - to tempt me into the fold - for $29/mo. but it would go up after a year to $49.
I work for a non-profit and we have three of us that use the programs I listed above to various degrees depending on the project. So we would have to pay $49/month for three users? I just can't justify that. I admit the day will come where we will probably have to spring for at least one but I will hold off as long as possible.
I don't like being locked into a $49/mo bill and getting upgrades whether I like it or not. There have been upgrades I skipped because they had issues. CS2 and CS4 were two. One required jumping through hoops to overcome sound handling issues and the other had unique problems handling some video encoding forms. I admit they were Premiere Pro issues. I have never had any issues with Photoshop upgrades.
I admit if Photoshop is the only app you need the $9/mo is not a bad way to go. at $108/yr it is cheaper since you no longer have to buy constant upgrades. If you are going to purchase it anyway that would be a cheaper option. I see that Amazon lists the cost for Photoshop CS6 for $1500???? Since I have always paid upgrade cost for the whole suite I have no idea what one app costs. Yikes!
The OP will have to make their own mind up. All they asked was what is the last version before the mandated cloud purchase version and we have answered that.