Best Alignment Protractor?

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.

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.