Ignoring Physics
If physics are not a requirement, then Halfthawed's answer (a cone) is the best answer (and I upvoted it). Maybe squish the back end of the cone so it's more oval than circle to bring a few more batteries to bear when the target is broadside rather than in front. When ignoring physics, the only best answer is the one that produces the most surface area during a primary attack.
A quick joke: A farmer wants the most efficient sheep pen possible. It must hold the most sheep for the least amount of fencing. The engineer says it's a rectangle because sheep are rectanglularish and so the most sheep can be put into the pen. The physicist says it's a circle because that maximizes area with the least circumference, minimizing fence cost. The mathematician, on the other hand, steps up, quietly draws a circle around his feet and proudly proclaims, "I declare everything outside this circle to be the pen." Why is the joke important? because this is the kind of answer you get when you ignore practical realities — like physics.
Not Ignoring Physics
1) Assuming all the batteries must be mounted on the ship, the best solution is a sphere. Indeed, when push comes to shove, the best solution for pretty much all space flight is either a cylinder (see L.Dutch's answer, which I also upvoted) or a sphere. The problem is control, which requires thrusters, and thrusters don't like dealing with angles. That leaves cubes, cylinders, and spheres.
Cubes are definitely Borg-ish, but the only way to get three of the six sides to bear upon a target is to point a vertex at it — which means those thrusters are dealing with angles and holding the ship on target is really hard.
The cylinder has the problem of batteries being one behind another in basically a line, which means unless you're always firing broadside, you really can't bring more than a fraction of your batteries to bear.
That leaves the sphere. No matter where your target is, you always have 50% of your batteries to bear. Sphere's are fearsome! Add to this the fact that they're much simpler to control via thrusters than any of the other configurations. You could always depend on whomping fast computers to do all the math to control thrusters ... but any engineer will tell you "KISS" (keep it simple, stupid!). The more you must depend on a computer to control your ship compared to the other guy, the more can go wrong that lets the other guy win.
However, mounting all your firepower on your ship is simply a really, really, really bad idea for two reasons:
A. First of all, a big, fat dreadnought is the poster-child for "hitting the broad side of a barn." They're great big slow (high mass) targets that are really easy to hit compared to their smaller counterparts. You could add more armor, but now you need more energy to move the darn thing. You could add more defensive weaponry, but again you need more energy. They're big, heavy, slow, and might as well have bullseyes painted on them.
B. And speaking of energy... Whether you're using yesterday's fission reactors, today's fusion reactors, or tomorrow's antimatter reactors... energy weapons demand a TON of energy at the moment they fire. That means lots of power generators and lots of batteries to hold that power — and that means any shot that hits your ship has a FANTASTIC chance of hitting said power generators or batteries... Foofh! no more ship. Most people don't realize that energy weapons mean generating a bazzillion times more energy during combat than at any other time. But holding all those generators/reactors at full operation 100% of the time "just in case" you need power is inefficient (see engineer example above) and dangerous. Batteries are a little better... but have you ever seen what happens when a big capacitor or battery is catastrophically discharged? Batteries are always dangerous. (Foofh!)
Space (in reality) is 100% about efficiency. Yes, the most daring, talented, experienced fleet commander will probably win the day — but only if the efficiencies of the two fleets/ships are about equal. Have you ever seen a small, trained wrestler wrestle a large, untrained dude? The little guy beats him 9 out of 10 times (actually he beats him 10 out of 10 times... but I'm trying to be nice). And to make things worse, if you choose a shape that isn't a sphere, then your dreadnought will always have a weak spot: the tail pipes.
If you want realistic you must be efficient. If you don't need to be efficient, who cares what shape your dreadnought is, because all that's left is aesthetics. Design something that looks cool and move on.
2) This leaves drones. You want those turrets mounted on drones and you want a million of them. The control ship is small and doesn't even need to be directly engaged in combat. Let the drones sit out there generating their little tushies off for all the power they need. Hitting a small drone with a single turret is a LOT harder than hitting your big ol' honking dreadnought and you can swing those buggers around to bring them all to bear in seconds-to-minutes.
This means your command ship is a flying computer with a maintenance bay, lots of antennae, a couple of bunks, and a cafeteria — but it's the most fearsome thing out there because drones don't need to protect human life (i.e., the lives of people onboard the drones—there aren't any). They can fly as fast as possible, be as agile as possible, and generate all the radiation they need to barbecue your enemies!
Conclusion
The shape of your dreadnought is irrelevant because what you really want is a lot of drones.
A quick note: WWII-era battleships are the dreadnoughts of our age, and the reasons I've listed are the reasons none of them are in service anymore....
