As it turns out, I own almost every D&D core rulebook out there. Basic, AD&D, D&D 3.0e, 3.5e, 4th and Next - they all site on a neat shelf (or in the case of Next, a neat virtual shelf on my Google Drive).
On top of that, in the last couple of years, I've had the pleasure of running D&D campaigns using Basic (to my kids) along with 3.5e, 4th and Next to my regular gaming groups.
Guess what - I really can't say which edition is my favorite. Each has its own pros and cons, each shine in certain aspects and performs rather poorly in other. I mean, you could run a great role-playing campaign in whatever version, but the rules, the setting and the story won't "synergize"- they won't combine into a whole experience that is both complete and sound (Godel must be turning in his grave now).
- Basic - simple but shallow.
- AD&D - definitely a step up, but most of its mechanics are awkward.
- D&D 3.x - Modern (compared to its predecessor), flexible, very detailed. Can easily be abused.
- D&D 4th - Cookie cutter MMORPG, but balanced and very easy on the DM.
- Next - Simple to run as Basic, with a meshup of features taken from 3rd and 4th.
Now paste your favorite setting (be it Forgotten Realms, Dragonlance, Eberron, Mystara, Darksun or whatever) over any of these system and things start to get shaky. Some are fairy compatible (AD&D and Forgotten Realms, D&D 3.5e and Eberron) but some can't be used together unless vigorous modifications are performed on either system or setting (Darksun and 4th, for example).
Which brings me to think that maybe - when deciding upon a new campaign - one should think about choosing a setting, and then choosing the system that bests describes it (maybe that's the reason that some many great settings come with a system baked right in - Eclipse Phase, Dark Heresy, Shadow World comes to mind).
But that leaves us with AD&D as a system - as most of TSR's campaign settings were designed using it (I'm assuming you're not building your own setting - a monumental task that really should be left to professionals IMHO). Let's face it - Basic had Mystara (which wasn't that bad, but wasn't that great either), 3.5e brought Eberron, 4th brought nothing (unless you consider Nenthir Vale as a campaign setting - and you shouldn't) and Next doesn't seem to grow from a specific setting.
So it seems to me that if you want that wonderful cohesion between system and setting, you need to choose the setting, and then choose the system that made it possible.
Here is a great quote by Monte Cook, which really encapsulate what's I'm aiming for. He wrote it about his home campaign setting - Ptolus - which was made possible by D&D 3.0:
The Ptolus Campaign is the d20 rules with the volume turned all the way up. I created this world with the game rules in mind. The conceits of the game were the conceits of the setting. The feel of the rules was the feel of the city. If the rules suggested that something might happen a lot, then in Ptolus, it happened a lot. The effects of 1st-level spells come as a surprise to no one here. Tanglefoot bags, rings of protection, and a druid’s animal companions are taken for granted.
I loved it when one day a player of mine said, “I polymorph myself into a troll and run out into the street after the thief.”
Another player said, “Dude, you can’t go out there like that!”
And the first player replied, “Don’t worry about it! This is Ptolus—they see this stuff all the time.” I knew then that the first player really got Ptolus.
Reading his monumental Ptolus Campaign Setting, I now realize what a good setting/system synergy is. Take a look at Eberron and 3.5e, or AD&D and Forgotten Realms. Maybe this is the reason these systems (and settings) were so successful - they were whole, and they provided a complete experience, with the setting build with the system in mind.
It makes me think what Wizards are planning for D&D Next. Provide books that tell you how to adjust your favorite setting for use with the new rules? It didn't work with 4th - it took them some time to release Darksun, which was made possible only by bending 4th just so.
So I really hope that they will provide flavors of D&D Next that will match the feel of the specific setting. Wanna play Forgotten Realms? Here's what you should add/remove from Next to support it. Wanna play Eberron? Here's how to decompose Next to fit that setting, etc.
Or they might surprise us with a new setting, one built with Next in mind...