What is the most vivid description of Hell.

This depends on what you find vivid. Here is one that I find very discriptive:

In the wide womb of uncreated Night,  

Devoid of sense and motion? and who knows,

"Our prison strong, this huge convex of Fire,
Outrageous to devour, immures us round
Ninefold, and gates of burning Adamant
Barred over us prohibit all egress."

"Rocks, caves, lakes, fens, bogs, dens, and shades of death,
A universe of death, which God by curse
Created evil, for evil only good,
Where all life dies, death lives, and nature breeds,
Perverse, all monstrous, all prodigious things,
Abominable, inutterable, and worse" Line 150-161