If we are going with what is currently available, then for me hands down has to be the Carson Mechanical tone. Can't stand Whelen's dying cow "Mechanical" tone, and the EQ2B also does not sound right.
I also agree with pdk9 that the Whelen Alpha Q's were the best sounding mechanical tone. It is unfortunate that the lawsuit came about and the best sounding mechanical tone was lost.
All time I would also have to include Federal Signal PA-5. While not labeled as a mechanical tone, the wail and manual tones mimic a mechanical siren extremely well. The yelp tone also sounds very close to a present day EQ2B yelp tone:
If I recall correctly this video, and several others (check out all the Scientific Prototypes videos!), were originally uploaded by an elightbars member. Again if I remember correctly, this particular PA-5 was rescued from a dumpster from a local police department.
Unfortunately I cannot remember which member this is, or if they even are still part of the boards.
Still have to give credit for an awesome series of videos!