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.

Best is the best. Mint Best Tracker is the easiest one I have used, sold the Feickert which is nice but just didn't need all the different options it and the others might provide. The Mint is specific to the arm/table combo, mirrored and so easy to use, after you get used to it you can align a freshly mounted cart in less than 5 minutes. Use with a good jewelers loop which is offered at time of order. Enjoy the music

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.

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.