there are 3 engine configurations that are inherently perfectly balanced:
* 60-degree V12
the typical American 90-degree V8 can be brought into perfect balance via the use of counterbalance weights.
A Ferrari V8 however is different. Although it is also a 90-degree V8, it has a flat crankshaft (i.e. the throws are all on one plane). Although not perfectly balanced, it has an even firing order (i.e. the engine pulses are regularly spaced), which gives it better high-rpm breathing and that distinctive shriek that cannot be mistaken for the typical American V8.
V6 engines are not perfectly balanced. I believe the 60-degree V6 is easier to balance than the 90-degree one. (benz went for the 90-degree config to leverage the same assembly line components of its V8 engines).
I view the term "boxer" differently: if you were to mimic with your forearms, the way the connecting rods of the opposing cylinders move, you look like a boxer using you arms as a shield.
A flat-6 has the lowest center of gravity and perfect balance. But its exhaust plumbing is very convoluted considering the small spaces available. And since it sits down low, it is more difficult to service.
An inline-6 has perfect balance but is the longest, which makes it more difficult to package. A long crankshaft is also inherently less stiff than a shorter one (like those on H6 and V6 configs), other things being equal. But intake and exhaust plumbing are simplified because they are on opposite sides of the engine. This makes turbocharging simpler than with a V6 or H6. You can also place heat-sensitive components on the cold (intake) side of the engine.
A V-6 does not have perfect balance but is very compact, and can be mounted transversely or longitudinally (the extreme example is the VW VR6). If mounted transversely, exhaust plumbing is a challenge. The space in between the banks simplifies the intake plumbing, and makes it a natural for superchargers, especially for a 90-degree V6, which has a bigger space in that Vee.
They also make different sounds, because of the differences in exhaust pulse patterns.
An inline-6 has a simpler valvetrain, because there is only one head.
Finally, an inline-6 looks the sexiest from the engine bay, what with that long *thing* in there, and with those exhaust headers in plain view ... it's a guy thing ... ;-)