I imagine it as a neverending bad dream. Think of all the ways that your own mind has been able to fuck with you. That but forever and with infinitely new and insidious ways to destroy what is left of your mental health.
Every day is a new way to be punished with visions of death and failure, maybe you'll see your girlfriend that hell convinced you was dead years(what even is time to a hell-resident?) ago, only to have her scream in pain and die as you embrace her, and you're left holding her entrails and skull, bits of her beautiful auburn hair stuck to your skin with her guts adorning the concrete as onlookers gasp and a new horror comes around the corner as you "wake up" only to have this next chapter more horrific than the last that you just "awoke" from.
It's a distorted, nonsensical land like the one YukiHerz described, but tailored to you. You're all alone in it, for interaction would offer some solace. It breaks you in new imaginative ways every single 'day" with methods and events that you have basically come up with for yourself.
Nobody else's worst nightmares are enough for you. You make your own. Hell is different for everyone. For some it includes monsters and demons chasing and prodding them forever, for others it involves the repeated loss and torture of loved ones or their own body, and for others it might be something worse than all of them. Maybe they force you to watch the slow destruction of a puppy and a little girl at the same time only for them to be fused into one ungodly hellspawn that you're forced to live with throughout the rest of your punishments, and then once you start to like it it is brutally murdered in front of you.
Anyway, that's my thoughts on what hell is. Well, actually, I don't believe in hell. I'm an asatruar; but I do write death metal lyrics, so this kind of imagination is good(?)
:D
(P.S.) Maybe hell is just hanging out with this dude and it's not even all that bad. God has a much higher kill-count than Satan after all. Ever hear of Satan's wrath flooding and killing millions?