Which aggregate (khandha) is or generates 'suffering'? When the mind or person experiences 'suffering' ('dukkha'), which 'aggregate' ('khandha') is producing this suffering or which aggregate does this suffering fall into?
I don't have the complete meditative experience or knowledge of the Dhamma to be able to fully comprehend the intents behind the teachings of the Five Aggregates and Twelve Nidanas, but I think I have some insights. I suspect that they are observable abstracted analysis of the complex working of the mind rather than actual entities within the mind itself - just as the body is only labeled 'form', but can be further analyzed in it's components such as the organs, or states of solidity, liquidity, gas and heat (earth, water, wind, and fire in ancient terminology). In this sense I believe the mental aggregates overlap in functions.

For example, the mind and mental activities in normal daily linguistic usage can be divided into feelings/sensation, thoughts, emotions, memories. Psychologists can probably further divide them according to various theories.

So Vedanā - commonly translated as sensations, is the observable feeling of pleasant, unpleasant, neutral when our internal sense organs come into contact with external sense objects and the associated consciousness.

Sañña - typically translated as "perception" or "cognition." Interestingly in Chinese this is translated as 想 - "thoughts". So this is our thinking mind

Saṅkhāra - 'volitional formations' mental formation etc. I think this generally the emotional response. The Chinese translation 行 has a meaning of 'action'/'execution'.

And finally the Viññāṇa 'consciousness' seems to be the catch all bucket for the general 'state' of the mind. You can have a pure consciousness, defiled consciousness and so forth.

As we can see, some functions of this overlaps, but the division help us focus our attention of where to comprehend things from.

In the Satipatthana.
The four domains are:
mindfulness of the body
mindfulness of feelings or sensations
mindfulness of mind or consciousness
mindfulness of dhammās.


  Satipaṭṭhāna  Ānāpānasati
  
  
  Contemplation of the body
  
  
  Breathing long (Knowing Breath)
  Breathing short (Knowing Breath)
  Experiencing the whole body
  Tranquillising the bodily activities
  
  Contemplation of feelings
  
  
  Experiencing rapture
  Experiencing bliss
  Experiencing mental activities
  Tranquillising mental activities
  
  Contemplation of the mind
  
  
  Experiencing the mind
  Gladdening the mind
  Centering the mind in samadhi
  Releasing the mind
  
  Contemplation of Dhammas
  
  
  Contemplating impermanence
  Contemplating fading of lust
  Contemplating cessation
  Contemplating relinquishment
  
  


The last one contemplation of the Dhamma is essentially a contemplation of all phenomena and of Truth.

The division of the mind into the four aggregates allow you to focus and observe the phenomena of impermanence, suffering and selflessness within them.

This is useful because we commonly suffer due to attachment to our feelings, thoughts, emotions, memories, consciousness.

The Buddha reminds us that all the aggregates are impermanent and hence subject to cessation, and hence attachment to all of them is suffering. They are not self because we don't have full control of them. But a positive way to look at it would also be that each of them is caused and conditioned, and we can try to create good causes to have good results. For example having good thoughts and enjoying the pleasant emotions and sensations resulting from that, without being attached to the loss of those sensations when the conditions ends.