Criteria: Level of Severity / Harshness, Fluidity During Gameplay, Difficulty and Uniqueness.

Realistic Needs and Diseases by perseid9

We suggest you use a mod that provides camping gear or a bedroll, and a survival mod that adds a need for sleep (It just so happens, that this mod adds all of them ). This will create the need to properly plan for quests and stressful situations when rations start running low. enhance the immersion of the Sleeping module. (It just so happens, that this mod adds all of them!)

FUN FACT: SoT comes loaded with sleeping encounters. Using a mod that makes sleep a necessary evil will ensure the occasional rude interruption. 

OR

iNeed by the masterful Isoku

This mod is a "lighter" needs based mod when compared to the above. However, the simplicity is its strong point. It allows "eating" to be hot-keyed so its not as cumbersome as looking through the menu for something to nourish one's self. Further, it adds "needs" to followers as well and can even have them join alongside the player when he eats.



Or, alternatively--if you prefer a more complex needs mod-- you may want to take a look at:

ZF Primary Needs by varakz

Imps more Complex Needs by Imp of the Perverse

Drink Eat Sleep Bathe by TheRealRyanB

Total Realism - Basic Needs by Mansh00ter

Use one of the above with this mod for a particularly tough time in Skyrim:

You Hunger by evrymetul

This mod aims to take a "needs" mod (such as listed above) to the next level by making food and drinks more difficult to find. When coupled with a needs mod, this makes preparing for your quest absolutely mandatory since you won't be able to count on endless all-you-can-eat buffets all over the place--the threat of starving to death is very real.

Complementary Mods: Since you are now required to prepare for adventuring, we suggest using this along with a mod like Inns Actually Cook to compensate for that lack of food. (Especially when using a needs mod that allows for follower needs such as iNeed.) This way, inns will have plenty of food in stock--just as they should. (NOTE: To properly install Inns Actually Cook, be sure to read the instructions carefully. Manual entry of a bash tag is required.)

And optionally:

Bathing in Skyrim by mz1n

A bathing mod you ask? On a list full of mods that make Skyrim a grungier, dirtier, darker, more demented and dangerous landscape? Yes. Do not question us peasant... Only be grateful for the bread-crumbs your overlords toss your way. Alright, in all sincerity, this mod is quite well done; it isn't over-the-top and makes one appreciate a settlement even more after a long dungeon crawl. We felt that the extra condition to keep clean was a "refreshing" and welcome addition. (Obviously if you choose one of the above mods that include bathing already in it, then you shouldn't use this.)

Complementary Mods: A mod such as Jokerine's Showers in Inns to accommodate stinking travelers.

Frostfall - Hypothermia Camping Survival by the great mod author Chesko

This adds a deep level of immersion making it so that cold weather is lethal to your character. Frostfall complements Realistic Needs and Diseases quite nicely and makes your travels across Skyrim much more dangerous. 

Survive the Night by Joubarbe

YES! Just simply...YES!. That's really all that needs to be said.
This most beautiful of mods does to night, what Frostfall does to weather. It makes planning for your travels an absolute necessity. If you are surrounded by darkness too long, you will start to experience fear. This fear will toy with your mind and tear at your sanity. The longer you let your self remain alone with your inner-demons, the more likely you are to succumb to a multitude of horrid visions.

NOTE: This mod makes traveling at night more dangerous, but it does not take into account if the player is a vampire or werewolf. For this reason, we do not recommend this mod if you plan on choosing one of those paths.

Battle Fatigue and Injuries by kuertee

Makes fighting wear you down (It'll "beat you down" so to speak.)--as it should. You will need to apply bandages and properly take care of your wounds after any intense battles. It is a fantastic immersion mod.

FUN FACT: One can double the bruising and abuse fun when paired with Bleed Them Dry (see "Gameplay Enhancements").

OR

Vigor - Combat and Injuries by Alex9ndre

An alternative to Battle Fatigue and Injuries. This is a newer, more streamlined approach, but the former still is unique in its own way. The two mods do almost the same thing, but handle how they do it differently. We list both here so that players are free to choose the one that better fits with their preferred play style.

Armors and Weapons Degradation MCM Edition by tonycubed2

This mod is an alternate and continuation of the mod Weapons and Armor Degradation by marcousik.

The mod allows higher immersion by introducing degradation of armor and weapons, including those provided by many other mods. Degradation can be caused by simple passage of time or combat.

NOTE: If you use this mod, make sure it is dead last in your load order to function properly.

OR

Loot and Degradation by Isoku

An alternative to the above listed mod which utilizes a different system for degradation.
Both mods have their advantages; choose the one you prefer and carry onward in a more brutal Skyrim young (or old--no offense) adventurer.

Artwork by GhostAgent

Skyrealism Encumbrance - As Envisioned by the SoT Team by tonycubed2

Ok you pack mule. No more carrying every weapon or piece of armor you find. Now, you will need to share the burden with friends and/or a horse (Convenient Horses) and stash your findings in a safe place for later use. This mod is based on the original Skyrealism Encumbrance mod and "SoTomized" by our team. (It's a more merciless version.) The original author gave us permission to take his beautiful idea and make something horrible and ugly out of it. Thanks!

Chasing the Dragon - Toxicity and Addiction by Ghenna

When the battle is beating you, do you find a hiding place and spam the health potions? Of course you do. You are weak, you are gutless, you are.....pathetic. We know it, and Ghenna knows it too. However, he has supplied you with the cold-turkey cure you need to overcome your cowardice. This wonderful little mod gives toxicity levels to potions as well as cause you to become addicted to them. Too much can lead to really bad side-effects (much like modern day prescription medications.) For a truly difficult game, use this along with a mod that increases spawns. It will discourage you from drinking potions rapidly--but not prohibit you from doing so (albeit with crushing consequences.)

A Friendly but Essential Notice for Newcomers:

Right now you look like a kid in a candy store seeing all those wondrous looking mods. We know, we get it. They are all awesome and that is why they are on our list. However, we feel as though we must inform you that the community's most respectable names in modding all agree that having too many scripted mods (pretty much everything listed above) is a bad idea and has great potential to break (your game) at some point. For that reason, they suggest using only a handful of heavily scripted mods at a time. (In other words, don't go overboard.)