Really, I'd have to say that for my usage and the breadth with which it can cover nigh anything, I would have to go with d20 Modern and its different manuals as the 'base' of any D&D project I'd like to take on. This is due to:
- Class Familiarity/Depth: Personally I hate the 'striker/controller/etc.' issue because Modern did it better. If you want a definitive 'easy to explain what they do' class set, you don't get easier than Strong/Fast/Tough/Smart/Dedicated/Charismatic. The depth comes in with the ease-of-build that are talent trees... sweet, simple talent trees.
- Has a 3e/d20 vibe, but can be more easily molded: Yeah, whenever you try to call up Al-Qadim or other 'lost' settings at the table you get crap because there isn't a Sha'ir/Clockwork Master/Zaharan Petal Merchant PrC. In d20 Modern the flexibility of the classes and the ease to 'port' prestige classes to advanced classes and vice versa combined with other materials... well, no issue.
Adjustments (Mechanical)
But d20 Modern's 5-level magic system sucks for 'epic fantasy'! And it's combat resolution mechanics, blah blah blahs...
And so, the 3rd Party materials come in:
-Blood and Fist: Great martial arts feats which, while missing a few D&D styles, can easily be adjusted. Also, the explanation of HOW to build new martial feats is so simple and down right in the text with balance discussions... good times.
-EoM: Mythic Earth: After all sorts of complaints and discussions, EoM:ME works quite well, and can be balanced based on other factors. Again, pretty easy to make new feats/adapt other traditions to the styles of your campaign's setting/world.
To resolve issues of heavy feat usages adjust feat and ability boosts.
Setting: Anything. Once I get a group and the setting is explained it's the choice of what we're going with, how we're adjusting, what WotC/3rd party is available, and PL and we're off to the races. Games have included 'classic fantasy', an Al-Qadim knockoff (because I didn't have my books in the house
), and numerous mixes of horror, adventure in strange environments, and (of course) modern fantasy.
Anything from 3.X/Modern or analogues can be torn up and tossed into the pot; I can create most anything I want with a decent knowledge of rules across-the-board, and my knowledge base expansion as DM makes for varied approaches which can be offered to my players. Really the best thing about Modern is the lack of setting baggage. You get to bring in any baggage you want, but you can insert at will.
But being able to create a wide and varied generic game, where a character can walk through settings from primitive to high future with small corrections. Little adjustments (use of Wealth and Reputation, addition of talents and races) have occurred, but overall it makes for a great base and development for my specific playstyle for almost any game.
Slainte,
-Loonook.