Who has the higher authority, the pilot in command or ATC?
It isn't a black and white issue of who has higher authority.  A pilot in command (PIC) is the ultimate authority for the safe operation of his airplane.  An air traffic controller is the authority for the block of airspace or pavement he controls.  

When you are operating under ATC, it is your responsibility to comply with their instructions as long as they fall within the regulations, they are safe and you are able to comply,  If ATC issues an instruction contrary to regulation or something that would compromise safety, that is when you exercise your PIC authority to deviate from their instruction.  If you are unable to comply, you exercise your authority to deviate and explain why you are deviating.  When you cannot comply with an ATC instruction, for whatever reason, it is the job of ATC to accommodate your and move other airplanes out of your way.

The regulatory basis for the PIC authority and ability to deviate from ATC instructions is in 14 CFR 91.3


  §91.3   Responsibility and authority of the pilot in command.
  
  (a) The pilot in command of an aircraft is directly responsible for, and is the final authority as to, the operation of that aircraft.
  
  (b) In an in-flight emergency requiring immediate action, the pilot in command may deviate from any rule of this part to the extent required to meet that emergency.
  
  (c) Each pilot in command who deviates from a rule under paragraph (b) of this section shall, upon the request of the Administrator, send a written report of that deviation to the Administrator.


The basis for ATC's authority is 14 CFR 91.123


  §91.123   Compliance with ATC clearances and instructions.
  
  (a) When an ATC clearance has been obtained, no pilot in command may deviate from that clearance unless an amended clearance is obtained, an emergency exists, or the deviation is in response to a traffic alert and collision avoidance system resolution advisory. However, except in Class A airspace, a pilot may cancel an IFR flight plan if the operation is being conducted in VFR weather conditions. When a pilot is uncertain of an ATC clearance, that pilot shall immediately request clarification from ATC.
  
  (b) Except in an emergency, no person may operate an aircraft contrary to an ATC instruction in an area in which air traffic control is exercised.
  
  (c) Each pilot in command who, in an emergency, or in response to a traffic alert and collision avoidance system resolution advisory, deviates from an ATC clearance or instruction shall notify ATC of that deviation as soon as possible.
  
  (d) Each pilot in command who (though not deviating from a rule of this subpart) is given priority by ATC in an emergency, shall submit a detailed report of that emergency within 48 hours to the manager of that ATC facility, if requested by ATC.
  
  (e) Unless otherwise authorized by ATC, no person operating an aircraft may operate that aircraft according to any clearance or instruction that has been issued to the pilot of another aircraft for radar air traffic control purposes.


Absent an emergency, an urgent need* or an inability to comply with ATC, a pilot is required to obey an ATC clearance.  Given this relationship and its exceptions, its is hard to quantify who has a "higher" authority.  Both are the ultimate authorities of their domains, but who overrules who is dependent on the situation.  In general however, ATC will be issuing proper clearances that a pilot is bound by regulation to follow, though the pilot must exercise his authority to execute that instruction in a safe manner.

*Urgent needs include things like TCAS RA events and things like weather deviations.  In most cases ATC will help you out with a deviation, but if a controller refuses to allow a deviation, I would happily exercise my PIC authority to safely operate my airplane and turn left/right a few degrees rather than penetrate a thunderstorm updraft.  



In the particular case above, the audio exchange between DL2422 and Atlanta ground is 


  DL2422: Good morning Atlanta ground, delta 2422 approaching 3 South taxi Quebec


DL2422 is at the southern exit of ramp 3 and requesting taxi with ATIS information Q.


  Ground: Delta 2422, behind the MD88 from your right, runway 27R, taxi via Mike and hold short of Dixie.


ATC has instructed DL2422 to let an MD88 pass from right to left, to taxi on taxiway M to runway 27R but to hold short for taxiway D (in ATL D is "Dixie" due to the larger number of "Delta" airplanes).  To follow this instruction, an airplane should taxi south from spot 3S on taxiway L5, cross L and then turn left on M, then taxi on M, stopping short of D and awaiting further instruction.


  DL2422: OK behind the 88 from the right, 27R Mike short of Dixie, DL2422


Here, DL2422 acknowledges this correctly.  


  Ground: DL2422 they won't let you go to Charlotte until 57 minutes after the hour, time now 29 after
  
  DL2422: OK copy 57, DL2422


DL2422 has been issued a wheels-up time, but their taxi clearance has not been amended.  They are still cleared to taxi via M short of D.  Sequencing to meet this time will happen east of D in further clearances.  


  Ground: DL2422, you are supposed to be on M  
  
  DL2422: Yes sir, we're going out there right now.  
  
  Ground: Looks like you joined L  
  
  DL2422: You know what, we'll taxi out there any way we want unless you tell us to, I don't like your attitude.  
  
  Ground: I don't have an attitude sir, I'm just saying it looks like you joined L instead of M and I'm just trying to correct you before you stay on L.
  
  DL2422: Ok, cause my god, there's another plane out there, its like 6 miles away.  Your attitude is really something sir, we're out here on M. Good morning.  
  
  Ground: Good morning.  There was no attitude, I was just trying to correct you, that's my job to correct you if you mess up.  I'm just trying to make sure everyone is doing what I ask them for certain reasons.  
  
  DL2422: You make a mistake every 2-3 minutes, but my attitude is not like yours, we're out on M and you didn't tell us how to get there so next time you can try doing that.  


The DL captain is making a scene and a very roundabout argument that the taxi route did not specify how he was supposed to transition from spot 3S to taxiway M.  I agree that this is the case, but do not understand why any assumption would be made other than to taxi straight out and turn left on M.  

This is a good example of a minor deviation that had DL2422 just turned right and then left and said "oops, fixing that, joining M, thanks!", then that would have been the end of it.  This captain's unprofessional display will instead net him disciplinary action and classify his deviation as "willful" rather than "accidental".  

For a clearer visualization of the event, see this image:

In this image, the green circle is the starting point of DL2422 and the blue line is the cleared taxi route with the assumed transition to M via L5.  The short red line is the initial taxi route DL2422 took.  It is not clear where they transitioned to M, so I did not extend the red route past the first possible transition.



There appears to be some controversy over the verbiage of "behind" used in the ATC taxi clearance.  The only reference I can find to this word in a cursory search is in FAA order 7110.65 (ATC procedures) in section 3-7-2 that gives example taxi clearances.  This does not define "behind" but does give distinct examples that clearly differentiate "behind" from "follow".  Given that I cannot find a better reference, all I can offer instead is years of experience in a 121 cockpit based at busy airports like IAH and EWR and plenty of visits to others like ORD, ATL, LAX, DTW, IAD, etc where clearances involving the words "behind" and "follow" are quite common.


Behind Use of this word means that you are waiting for an aircraft to pass.  You are giving right of way or yielding to the traffic in question.  This clearance is often given when holding short of an intersection and waiting for perpendicular traffic to cross the intersection before you continue through it.  Much like waiting at a stop sign in a car.  You are not expected to follow the airplane in question and even if you are assigned a turn to taxi along the same route as the airplane, you have your own clearance and are not following that aircraft.  

Examples:


"Continental 1234 taxi to runway 22R at W via K hold short of S", followed by "Continental 1234 you'll see a B767 passing right to left on S, behind him continue to runway 22R at W via K R hold short of W"


In this case it is quite explicit that an airplane will be passing across your route of taxi and after it passes you are to continue taxiing straight ahead -- not to follow that airplane.  Consider an alternate setup:


"Continental 1234 taxi to runway 22R at W via K hold short of S", followed by "Continental 1234 you'll see a Citation from the right at S, behind him taxi via S W hold short of R.


In this case, you are waiting for traffic from the right again, but this time you are turning left after it passes.  You will be taxiing behind it, but you are not following it.  The Citation has a clearance to taxi S, cross runway 29, to parking.  Your clearance is S turn right on W -- to follow the Citation would be wrong.
Follow Use of this word means you are going to fall in behind this airplane and follow it wherever it goes.  You clearance is no longer a specific taxi route, but simply to follow the airplane.  

Examples:  


"Jetlink 1234, your sequence is the third airbus to your right, the one painted purple.  Follow that airplane to runway 22R at W.  Once behind him, monitor tower, you are number 28 for departure.", which will be followed by
"United 3412, hold short of J, let the EMB-145 get in line then follow him to runway 22R at W.  Monitor tower passing L, you are number 29 for departure"


These clearances are used to get airplanes in line in a specific sequence and simplify giving repeated complex taxi instructions when you can just follow to conga line to the runway.