Superb poll Matt!
I'm going to go with the left-field choice of the Boxer engine here of course. Ok, so I'm being a little too predictable eh?
Boxer engines have a great many virtues and only a few flaws.
Like you stated, Boxer engines have very little vibration courtesy of the horizontally opposed (and hence counter-balancing) piston arrangement. Boxer engines can hence be made much lighter (they're more rigid) because they don't require ancilliaries light counter rotating balancer shafts. Low CofG is a great plus too Matt, like you said.
Boxer engines have a short-stroke so smaller capacity NA engines can be a little short on torque, even though the spread is fairly even. But, the short stroke has many advantages: mean piston speed is lower and so with a little bit of engineering (like titanium con-rods in a GT3) they can be made to rev high without compromising reliability. Boxer engines just love to be turbocharged and the results are nothing short of spectacular.
Boxer engines' biggest drawback is the lack of application versatility. A transversly mounted in-line four is a far better packaging option for compact fwd cars for example. It's not like you'll ever see a great hot hatch like a Clio 182 with a boxer engine now eh?
And, a straight six or V arrangement is better suited to rwd applications if the engine is mounted in the front. Boxer applications require very specialised and unconventional installations.
Other than that I believe a straight six to be one of the purest engine layouts around. Love em.
V's are great too, specifically because they're the only way to achieve truly large displacements and hence max power - see AMG's new 63 V8 and BMW's V10s by way of example. Obviously, such power comes at a price. V engines are expensive simply because they're bigger and more powerful.