The "best" cartridge alignment tool is probably the Wally-Tractor, which is custom-made for each particular tonearm/turntable combo. I think it costs about $150 from Wally, and takes a couple of weeks for an order to arrive.
However, the Dennesen Soundtractor (which was made in both metal and plastic versions) is very good (I have the original metal version, which I use to set up my Rega 900 tonearm and Grado Reference cartridge). I think the plastic version has recently gone back into production, but if you can find a used metal version, it's certainly worth having.
The other possibility is the Geo-Disk mat, which works pretty well and is easy to use.
However, the Dennesen Soundtractor (which was made in both metal and plastic versions) is very good (I have the original metal version, which I use to set up my Rega 900 tonearm and Grado Reference cartridge). I think the plastic version has recently gone back into production, but if you can find a used metal version, it's certainly worth having.
The other possibility is the Geo-Disk mat, which works pretty well and is easy to use.