Which makes the biggest difference?

@ghdprentice says it well..

You don’t want the rest of the signal chain down.

so which is the most important link in a physical chain? hmmmm

No clue what I had intended that sentence to say.

How about: You don’t want that signal to go down to a weak amp or inferior speakers either.

While I can make rationalizations for why each component matters, particularly those at the front, the reality is they all matter. If you have a weak link, improving the weak one can make a huge sonic difference. But when analyzing the problem, it is best to keep in mind the front end and preamp as having additional weight in the analysis.

OP's questions leads us to a game of Six Degrees of Audiogon Thread Separation. Can't talk about amp impact w/o mentioning the speaker in use. Can't rank preamp above or below DAC without discussing analog vs. digital volume controls and relative quality of them for the DAC and preamp in question. 
Please refer to EVERY DARN THREAD EVER POSTED for the complete picture. JK. 
Can I add "the mastering of the recording is most important"? 
No? Fine. It's the DAC. Cheers,
Spencer

@ghdprentice

haha - i quoted your earlier statement fully understanding what you meant to say!

it is a chain, every component is a link... so the weakest link is what matters the most!

that is the fun (and challenge) of this pursuit... figuring out what that is in one’s own system... and then the weak link will usually move once you make a change!

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and spencer, i agree 100% -- the original recording quality matters one heck of a lot

that is one pivotal piece of ’strategic planning’ in system building, if you will -- one has to honest about the range of recordings one will use the system to listen to, and then the system needs to be built to maximize enjoyment of that range of recording qualities to be played on a regular basis