Who seems most devastated by Juliet’s death?

In Shakespeare's play Romeo and Juliet, Juliet actually dies twice, once faked and once by her own hand. After taking the vial of potion given to her by Friar Laurence in Act IV, Juliet appears dead to those who discover her, including the Nurse, Lady Capulet, Lord Capulet and Paris. In most performances of the play this is a very powerful scene as all four characters express their grief at the same time and you can hear one voice on top of the next. 

Since she has been closest to Juliet for most of the girl's life the Nurse is quite distraught after finding Juliet's limp body. In Scene 5, she says,

O woe, O woeful, woeful, woeful day!
Most lamentable day, most woeful day
That ever, ever I did yet behold!
O day, O day, O day, O hateful day!
Never was seen so black a day as this!
O woeful day, O woeful day!

Lord Capulet has lost his only child and his despair is particularly poignant since he had been planning a "day of joy" for his daughter in the aftermath of the death of her cousin Tyblalt. He says,

Despised, distressèd, hated, martyred, killed!
Uncomfortable time, why cam’st thou now
To murder, murder our solemnity?
O child! O child! My soul and not my child!
Dead art thou! Alack, my child is dead,
And with my child my joys are burièd.

In similar fashion, Lady Capulet expresses much of the same grief as her husband. She says,

Accursed, unhappy, wretched, hateful day!
Most miserable hour that e’er time saw
In lasting labor of his pilgrimage!
But one, poor one, one poor and loving child,
But one thing to rejoice and solace in,
And cruel death hath catched it from my sight!

Paris too joins the mournful chorus. Juliet, after all, had been betrothed to him, and even though he did not really know her well, we feel his grief is heartfelt, especially after he risks his life to apprehend Romeo at Juliet's tomb in the final scene. He says,

Beguiled, divorcèd, wrongèd, spited, slain!
Most detestable death, by thee beguiled,
By cruel, cruel thee quite overthrown!
O love! O life! Not life, but love in death!

Even though her family is plunged into depression over Juliet's supposed death, the character who takes it the hardest is certainly Romeo. As soon as Balthasar tells him of the bad news from Verona, Romeo begins plotting his suicide: "Well, Juliet, I will lie with thee tonight." For Romeo, suicide is the ultimate declaration of his love for Juliet. Unfortunately for Romeo he fails to recognize the signs that his love is simply under the influence of a strong narcotic and still lives. He says,

Thou art not conquered. Beauty’s ensign yet
Is crimson in thy lips and in thy cheeks,
And death’s pale flag is not advancèd there.

Finally, both the Prince and Lord Montague express their sympathy over Juliet's death. The Prince understands death all too well because he has lost two relatives in Mercutio and Paris during the violence of the feud. Lord Montague even suggests he can do more:

But I can give thee more,
For I will ray her statue in pure gold,
That whiles Verona by that name is known,
There shall no figure at such rate be set
As that of true and faithful Juliet.