Best Alignment Protractor?

The best depends on whether you prioritize ease of use, accuracy, variety of alignment schemes, or universality (can be used with any arm).

I particularly like the SmarTractor for ease of use and variety of alignment schemes offered; it is recognized as being very accurate when used properly.  The ease comes from having to only make sightings and placing the stylus down on the protractor at one single point.  The only downsides are that it is not universal because one must know very precisely where is the pivot point of the tonearm and that point might be difficult to find on some arms, and there are no alternative measurements to confirm that the alignment you set is good.

The Wally Tractor is more universal (pivot point of the arm does not have to be precisely determined.  It is harder to use because one needs to make more sightings after precisely locating the stylus at measurement points or along a measuring curve.  But the multiple measurement points offer more confirmation that you have the correct alignment.

The Feikert lies between these two—it is not as easy to use as the SmarTractor, but easier to use than the Wally—and it offers some confirmation, but not quite as clearly as the Wally.

If I had an arm with a precisely known pivot point, particularly one with a dimple at that point where the protractor pivot point indicator can be placed down in that point, I prefer the SmarTractor.  If I were obsessive about perfect alignment I would choose the wally.

I agree that a custom mint is an easy and accurate protractor.  It has to be custom made, and you must know the precise spindle to pivot distance.  I don't know if I would rely on the turntable manufacturer having precisely drilled the hole for the arm and I would therefore do a measurement.  Unfortunately, a really good measurement device for doing this is not easy to come by (the SmarTractor includes such a precise measurement device).  I think the mirrored surface of the mint is quite easy to use to properly align one's eye to do the reading (same thing as the SmarTractor and the Wally Tractor, but, for some reason I've heard of complaints about difficulty in reading the Mint (I don't know what this is about).

All of these alignment tools are best used with some sort of low powered magnifier to more easily see the point where the stylus touches down and to see how the cantilever aligns with the line inscribed in the protractor.  It also helps to have a decent directional light source.

The Mint is also MUCH cheaper than the SmarTractor or the Wally Tractor.

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.  

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.  

I agree that high-precision protractors and ultra precise alignment makes for sense for a dealer than a home alignment.  The big problem is that no matter how precise the alignment, built-in errors in the manufacturing of styli, cantilevers, and cartridge suspensions pretty much dwarf minor errors in user alignment of the cartridge.  Look up zenith error.  Even with extremely expensive cartridges, zenith error can be very high because it is really hard for the manufacturer to spot the error when mounting the stylus.  The kind of microscope required to see and measure zenith error cost as much as a decent automobile.  Unless you have a conical stylus, zenith error will put the wrong part of the stylus in contact with the groove wall which would mean more wear on both the record and the stylus.  An analysis of zenith costs big money, but, it will allow one to justify seeking correction for gross error from the manufacturer or making compensatory adjustments for more minor error (correction would involve twisting the cartridge away from perfect alignment of the cantilever with the line on the protractor).

Some of the other alignment measurements people take also don't make that much sense, such as insisting on a 92 degree stylus rake angle.  This is only roughly the correct angle under dynamic (while record is in play) conditions.  If you make a static measurement using a USB microscope, you will not be setting the angle to what it will be while the record is playing.  Again, there are complicated ways of measuring the angle under dynamic conditions, but, one is really better off just setting an approximate angle (tonearm perfectly parallel to the record surface) and then listening for improvement in the sound from trial an error deviation from the approximately correct position.