Best Alignment Protractor?

I have used the simple plastic protractors that SME provides with the Series V and M10 arms, and a Dr Feickert (where I believe the Baerwald configuration is about the same as the SME set up). I can't really tell any difference on listening though.

The specialty protractors or alignment devices for specific arms can be the easiest to use.  The SME is very easy to use, particularly because you move the arm systematically until you get the right distance (not as much guessing and overshooting the mark).  The one that came with my Vector arm is also pretty easy to use, though not quite as easy as the SME.  The ultimate in ease has to be the Graham arm because the alignment is done with the headshell completely removed from the arm and the headshell is then inverted so that one can easily see the cantilever and the stylus point to make the alignment.  

An old plastic Dennesen Soundtractor. Yeah, the part of the device that extends from the center hole to the tonearm's pivot point broke at some point in the distant past, but it was easy to accurately put it back in working order with a couple windings of Scotch Tape. Luckily, the component broke in such a way that it was a cinch to reassemble it to audiophile tolerances.  To be sure, it's important to wear magnifying glasses and not to drink too much coffee before mounting cartridges as you don't wanna have the shakes, but I've always done damnably accurate work.  Depending on the shape of the stylus, of course, and vinyl pressing vagaries, I  get near flawless tracking from edge to lead out groove with the vast majority of the records I play.

DBP-10 (DB systems) has served me perfectly over the years.

Is it the best for convenience? Not really but it is accurate, that matters the most, and very important it does not cost that much.

Learn how to to take the max out of your protractor, that would be the best one.

Even the one supplied for my V arm is excellent if you know how to use it.

I have no idea just how much precision is required to get the most out of an arm cartridge combination.  Obviously, it doesn't hurt to optimize the geometry, but, it is not clear to me how much is lost by being slightly off.  This is particularly the case when one finds out how much imprecision that is built into analogue playback in so many other respects.  You can align the cantilever precisely with the lines on the protractor, but, when in play, forces, such as skating forces, pull the arm out of alignment.  Perfect alignment only optimizes geometry if the zenith of the stylus is perfect (i.e., if the stylus is mounted on the cantilever such that the wedge of the stylus is perfectly 90 degrees to the cantilever edge).  It turns out that it is very hard to get the zenith correct and many very expensive cartridges have styli that are misaligned.  The only way to sort of compensate for this is to have the stylus examined under a very specialize microscope and then deliberately mount the cartridge with the cantilever out of alignment enough to compensate for the zenith error.  This is extremely complicated, and can only be analyzed by specialty companies, like Wally Tools.

There are even tonearms on the market with NO offset angle at the headshell, so they end up having massive errors when it comes to aligning the cantilever as close to tangent with the groove.  The theory behind such arms is that this geometric error is of less importance than eliminating the skating forces caused by an offset angle.

The upshot is that being ultra-precise about alignment may not have that much practical benefit, and it might well be the case that a cheap protractor with thick, imprecise lines and no way to establish the perfect viewing angle is still good enough.