Which poisons do you think are useless? Does anyone use fear or lingering poisons since they are so weak? What's even the point of having them in game?

I craft my own poisons, and on my second play through I have not put perk points in the poison side of the Alchemy tree because crafted poisons are too powerful.
Likewise, I also avoid making damage health type poisons because it kinda stops the game. Everything dies to the self made poisons after one hit.

The poison I use is a Paralysis-Damage Stamina poison. That way the poison is very satisfying when you sniper a bandit, say, and they fall over paralysed, but the poison cannot kill them. Paralysis is also essential for assassinations, so if you mess up your kill in one hit, the victim will not make any noise as you finish them off.

Other poisons may provide a bit of entertainment, like Frenzy and Slow, but I never use them.

I only use damage health and paralysis poisons - everything else gets sold.

Lingering poisons are NOT weak, not when you get alchemy up a ways. I think mine do 10 damage/sec for 25 seconds or some such, and there's a couple perks I could take to make them even stronger.

Hit a dragon, giant, or mammoth with a couple of those, and you can just run away and hide, coming back to loot the corpse shortly after.

Especially good when combined with paralysis. Here's some recipes; the "Restore Health" effect in some of them doesn't matter (assuming you haven't done any other damage yet) because it kicks in before the "Lingering Damage."

Slaughterfish Scales, Imp Stool, Canis Root Lingering Damage Health, Paralysis
Briar Heart, Imp Stool, Orange Dartwing Paralysis, Lingering Damage Health
Briar Heart, Imp Stool, Slaughterfish Egg Paralysis, Lingering Damage Health
Imp Stool, Canis Root, Mora Tapinella Paralysis, Lingering Damage Health
Imp Stool, Canis Root, Orange Dartwing Paralysis, Lingering Damage Health
Imp Stool, Canis Root, Slaughterfish Egg Paralysis, Lingering Damage Health
Imp Stool, Swamp Fungal Pod, Mora Tapinella Paralysis, Restore Health, Lingering Damage Health
Imp Stool, Swamp Fungal Pod, Orange Dartwing Paralysis, Restore Health, Lingering Damage Health
Imp Stool, Swamp Fungal Pod, Slaughterfish Egg Paralysis, Restore Health, Lingering Damage Health

I've never used the frenzy poisons, as I think they might get things worse for you in certain circumstances...
I rarely use the weak lingering poisons, only when I don't see an immediate chance to sell them as an alternative way to get rid of them.
I've discovered lotus extract, it's quite cool.

I've been thinking about using frenzy poison in the Dark Brotherhood Quest line to kill the bride. Maybe she starts slaying guards then and is killed by the imperials... I would be a coward then, but at least an evil one... :-p

Poisons kind of fell of the wagon for me.

I never felt much of a need for the poisons. I played around with them, but they are gone after one hit. If I could poison a blade and use it for 15-20 hits then I could be more interested in them. They are just too much effort, from making them, to carrying them around and to applying them. You cannot create poisons everywhere and you will want to use special gear and the weight of 0.5 for a single bottle seems a little too high for all that, too. The effort it takes to make one poison, to apply it to a bow and thereby deal some extra damage stands in no comparison to the effort of simply shooting a second arrow. Playing as a poisoner feels unsatisfying. Smithing and enchanting gives more damage and are more lasting than poisons.

I think the slow poisons are pretty useless, they still seem to get to me just as fast. I like pickpocketing the frenzy poisons into bandits, that can be quite fun. Pity it only lasts about 10 seconds though, it's barely enough time for them to do any damage before they stop and say "Sorry bud, friends again?"

But this seems like a good place to ask rather than create my own thread, but does anyone know what is the difference between normal damage health poison and the ravage health poison?

The damage magicka and stamina have no use. Dragon priests cannot be harmed by them, so no need for magicka and damage health or lingering damage health are better anyway. if there is a mage that is affected by poisons, but not killed easily, then paralyze would be better.

Ravage health (aka concentrated poison) reduces the maximum health and is useful against fast healing opponents, i.e. mages who use healing spells. It is pretty useless, imo.

I don't think I've ever use poisons in combat. I've had a lot of fun sneaking them into people's pockets though.

I've never found them to be very effective and I've gone pretty high up the alchemy tree. I even saved the supposedly deadly Jarrin Root from Muiri to use on bigger, badder opponents but the hoped-for effect didn't happen.

Damage Magicka Regeneration isn't any good in combat either but it I make lots of them because of the $.

The ones I consider "useless" are all the random ones I find. Paralysis, damaging magicka, etc... that's pretty meaningless when you just need someone to die.
I only use the ones I make.

I agree that poisoning is too much trouble and the weight of 0.5 makes me choose to carry helpful potions instead. I forget about them most of the time, to be perfectly honest. I mostly just sell the poisons. I do use health, magicka, and stamina potions in battle, especially with boss enemies.

I only use them as decorations.

Damage magic/stamina are pretty useless, I don't bother with those. Slow poison is pretty situational as well, but it's better than damaging stamina/magic. As for the rest:

Frenzy poison- Perfect assassination tool (or just for lulz at bandit/forsworn camps). Pop one in someone's pocket and it's free reign to kill them without consequence.

Paralysis- Great for thieves, as you can paralyze someone and then pick their pocket and get a 100% success rate to grab anything without getting caught. I've stolen stacks of 40k+ gold. Great for combat in general as well for guaranteeing second chance success if the enemy doesn't go down in one hit. Also lets you incapacitate one enemy and deal with more.

Damage health- Pretty straight forward, much like tacking on extra damage to your hit. Good for higher leveled beasts you know will take multiple shots to bring down.

Lingering damage health- More powerful than damage health, but you have to wait a bit longer for results. Great for stacking damage against an enemy since you can attack them while their health keeps draining from the poison.

Ooh... The Paralysis Poison can make guarantee success with pickpocketing? I'm gonna have to try that with Cicero's clothes.

I only use the health damaging poisons and the paralysis poisons and even then only rarely.

I like the frenzy potion for certain DB targets. Otherwise I'm in agreement with what has been said already. However, I tried a paralysis poison on a target and was unable to pickpocket him while he was paralyzed. Only before and after the effect could I pickpocket him. I haven't tried since then.

Damage Stamina and Damage Magicka I never use...However, I never actually leveled up alchemy, I should try it out in my next gameplay...I hear the poisons and potions become amazingly powerful...

Pretty much my thought as well.

I don't really use poisons because I don't have many
I think i used the Damage Magica/Stamina few times just to try it out. I liked it

I have many poisons but I just like to tollect them all so yeah I dont really use them for now.

I created a stamina poison using the fortify restoration glitch and it gave me a stamina poison of well above 4000. I used it on an arrow on an enemy I knew I couldn't one-shot, but he still came at me just as quickly and used power attacks. So stamina poison gets my vote as the most useless type.

I reckon the magicka poison good against mages, but I think they'd soon recover it. Or use a staff on ya.