interesting question. and one i've personally been trying to answer for 4-5 years. and i don't yet have the answer.
my first deck was a 1/4" ATR-102 completely redone by Jeff Gillman of Precision Motor Works. this deck also came with a 1/2" head stack.
then i acquired a pristene condition 1/4" Studer A-820 that was redone by Fred Thal.
then i added a very clean 1/2" Studer A820.
recently i purchased an Ampex 350 in traveling cases, which is a virgin and super clean, but in need of a thourough going thru, which Dave Dintenfass will be re-doing for me.
and lastly i've acquired a very clean Nagra T which should be in my room next week.
at various times all the above machines have been characterized as the best from one perspective or another.
currently i have one set of custom output electronics, the King Cello. i've also purchased a pair of Ampex 351 Mic pre's, which i'm planning on using with a transformer for the low impedence modern heads....once Dave get's those done. i'd like to add one more set of output electronics.
i've been 'told' that the Studer A-820, A80, and Ampex ATR-102 are the very best transports in terms of tape handling and speed accuracy. heads seem to be a matter of taste; different people view those differently based on a sonic compass.
many tape decks are judged by their native output electronics; which mostly are crap, with a few exceptions. some love the tubes, others not.
which one is best? as a machine and considering build quality; there is the Studer A-820 and then there is everything else.
just one man's opinion.
