Wow, this is a tough call and a really thought-provoking topic, Mag...
I've heard a really good preamp take what is theoretically a top-drawer system and kick it to the next level, I've heard the difference an uber-front end makes in a well-considered system and I've heard what a difference a pair of really good speakers brings to the table. Put 'em all together and it's the total package but is there one piece that is the deal-breaker? I think I'd have to go along with the thought that the sum is greater than the individual parts but that feels like a dodge of the question in the OP.
Gun to my head, I guess that the component in my system that connects with me as being less of an
appliance than the rest would be my Manley Neo Classic 300B pre. It's such an esoteric thing ( and I have an
actual Esoteric thing in my system...

) but it has an odd charm and really, really does good things for my system as a whole. When my new speakers finally arrive though, they may usurp the throne because I've heard those things in this system and they are a fantastic addition.
D.D.