There are a lot of "Have To" mods for me. The top three I must have for any character I play are...Well, I will cheat...

1) Appearance
Realistic Lighting - http://skyrim.nexusmods.com/downloads/file.php?id=7654 in conjunction with,
The Imaginator - http://skyrim.nexusmods.com/downloads/file.php?id=13049

These two will set the atmosphere. Just running the "default" of RL, to get the dark nights, dungeons and interiors, along with the Imaginator to tweak the whole scheme to what brightness and color effect I want, really makes the game incredible...visually and changes the way you play with the dark. I actually have to use a torch, now.

I spent an hr of real time ( I adjusted the time scale so days are longer ) getting from the exit of BFB to Riverwood. My last torch burnt out just as I left the barrow. I do not fast travel anymore and run with no compass. It was an overcast evening and completely black. I could just make out the outline of the Throat of the World in the background. That was my guide. It was intense!

2) Functionality
Sky UI - http://skyrim.nexusmods.com/downloads/file.php?id=3863 in conjunction with,
Immersive HUD - http://skyrim.nexusmods.com/downloads/file.php?id=3222

These two are a MUST for the game now. They provide a better system and gameplay. Great stuff indeed.

3) Gameplay - This one can get real crazy real fast with a TON of mods.
PISE -http://skyrim.nexusmods.com/downloads/file.php?id=1269 in conjunction with,
Deadly Dragons - http://skyrim.nexusmods.com/downloads/file.php?id=3829

These two are, so far, working great together. Be warned it is absolutely brutal! But that is what I wanted. Truly epic mods. No more soft milk drinkers here...

Holding to just three is not fair. There are many, many more to be used as well. Ones that add dialog, quest descriptions, improve followers, weapons, armor player houses...the list is absolutely huge.

Glowing Ore Veins: http://skyrim.nexusmods.com/downloads/file.php?id=193

Alternate Start: Live Another Life: http://skyrim.nexusmods.com/downloads/file.php?id=9557

Bobs Armory: http://skyrim.nexusmods.com/downloads/file.php?id=12060

http://www.darkcreations.org/forums/files/file/205-unofficial-skyrim-patch/ - A ton of other fixes and overall stability. It basically should be baseline.

http://skyrim.nexusmods.com/downloads/file.php?id=9959 - Probably the best way that controlled my tendency to powerlevel through smithing, as well as forcing me to use different weapons and gear through all my download packs instead of just focusing on the best ones. Plus it reminds me of WoW with the recipe books (I'm hoping for a way to have different drop rates for different 'tiers' of gear). Add it in with the other 'hardcoe' mods and it basically makes progression a lot more satisfying and varied in Skyrim, extending the lifetime of the gameplay.

http://skyrim.nexusmods.com/downloads/file.php?id=7654 - Baseline because Skyrim was bloody bright and felt like the gamma was set way up; even helps with FPS because removal of ambient light and adjusting to realistic positions more often than not gains you a bit of FPS.

Threads like this are really good, because they can help other people notice good mods that they have missed themselves.

I don't use mods atm, but if I did use mods, my top three would probably be:
Better Quest Objectives,
Water,
Realistic Overhaul.

1. Deadly combat
2. Basic human needs.
3. Hypothemia

Top 3 but there are now about 40 mods which have become essential.

Strictly going by the topic "mods you can't live without", these are the mods I would not start a new Skyrim game without:

1) Glowing Ore Veins (http://skyrim.nexusmods.com/downloads/file.php?id=193)
2) tejon's Simple Skyrim Spell Scaling Solution (http://skyrim.nexusmods.com/downloads/file.php?id=2585)
3) A Quality World Map With Roads (http://skyrim.nexusmods.com/downloads/file.php?id=4929)
4) SkyUI (http://skyrim.nexusmods.com/downloads/file.php?id=3863)
5) Reduced Distance NPC Greetings (http://skyrim.nexusmods.com/downloads/file.php?id=746)
6) Silent Dog Idle (http://skyrim.nexusmods.com/downloads/file.php?id=1579)
7) Conjuration and Summoning Fixes (http://skyrim.nexusmods.com/downloads/file.php?id=3352)
8) Some sort of mod that removes +smithing/+enchanting effects from potions and +alchemy/+smithing effects from enchantments. Last time I used a custom-made mod adapted from my Less Overpowered Alchemy/Enchanting, but next time around I'm thinking I'll try Phitt's Alchemy and Food Overhaul (http://skyrim.nexusmods.com/downloads/file.php?id=11481)
9) Haven't played with it yet, but something that I suspect will be essential from now on is SHARPE Made to Order (http://skyrim.nexusmods.com/downloads/file.php?id=14474).
10) Also haven't played with it yet, but likely to become essential: More Interesting NPCs (http://skyrim.nexusmods.com/downloads/file.php?id=8429).

Ok, so that's 10, but to my mind those are all pretty much essential and fix serious problems in the game (really, all of those mods are fixes in one way or another). I play with a bunch more mods than those, but those I would not play without.

In no particular order...these will always be on my mod list

1) Live Another Life
2) Skyrim Monster Mod
3) Real Wildlife
4) Sounds of Skyrim (Wilderness+Dungeons)
5) Realistic Lighting by Plutoman
6) CBBE, with heavy tweaks to Nidia face textures and slight edit to Nord race
7) Frostfell
8) Imps More Complex Needs
9) Vampire FX + Vampire Overhaul
10) Tales of Lycanthropy - Werewolf Overhaul
11) HLP Night Sky (amazing night sky mod, stars even twinkle!)
12) Deadly Dragons
and finally...13) Occupy Skyrim - OCS

I find it interesting that nobody mentioned "http://skyrim.nexusmods.com/downloads/file.php?id=210"

It's one of the most important fix in Skyrim history.

Although to be frank, I've never taken that off. Perhaps later version of Skyrim removes this?

For me, the second would be SkyUI, yes, and the third would be Quality World Map (with roads)

In no particular order ..

USKP http://skyrim.nexusmods.com/downloads/file.php?id=19

and .. erm, thats it for now

Although maybe one additional one for performance

Skyrim Performance Plus http://skyrim.nexusmods.com/downloads/file.php?id=6387

was a pretty good idea ...

For me:

- Realistic Lighting w/ Customization

- The Dance of Death (killcam mod)

- SMIMs ( Static Mesh Improvement Mod )

No matter how many times these types of threads come up I always come away with a new interest in a mod I didn't know about. Today it was Tales of Lycanthroy. Thanks!

My top 3 have been mentioned already.

Wait... Did my thread get merged? o,O

If so, can a mod incorporate my post into the original as people are answering similar but different questions!

It did get merged. I have two posts answering two different questions in the same thread.

I agree with DeSetuede: if two questions are to be merged into one thread both questions ought to be included so we can make some sense of the answers.

Only three? Okay, I'll go with:

ASIS
Community Uncapper
Skyrim Improved Magic Experience

Effeminate Nazeem didn't quite make the cut. There are many others though that are also indispensable, like the unofficial patch and Realistic Lighting, but I went with favorites.

My three are:
Duke Patrick's Heavy Weapons Combat
Frostfall
Darwins Nightmare

Honorable mentions include:
Super3l ENB (or whatever it's called)
Handyman
SkyTEST - Realistic Animals & Predators
Deployable Traps
Occupy Skyrim
Warzones

I don't think I can live without any of my mods. Just the game became interesting again.
If I have to name the main ones -
Artefact & Enchantment Balance Overhauls
Boyds Better Ballance - Autoperkpoints, LowerStartingSkill and Perks
Deadly Combat
Deadly Dragons
Frostfall and IMCN
HBE
Phitts Alchemy
PISE
UFO

and last but most important - R-key Drop Items Fix

I wasn't aware of either of these, and they look fantastic! Thanks.


Was also unaware of this one, which looks potentially interesting.

Thought I was going mad then o,O lol...

AV - Automatic Variants - http://www.gamesas.com/topic/1364221-relwip-automatic-variants/

Skyrim is so boring without it

Sounds of Skyrim - Wild and Dungeon - more immersion.

Dragon Overhaul - makes dragon not as piss easy are they are in vanilla

Can you post yours again, yourself? One issue with mods seems to be catching their attention...

SkyUI:
Fantastic "must have" for a better UI.

Deadly Dragons:
I love how it makes a key part of the game much more interesting.

Racial Power Auto-cast:
Granted, I may be biased; but I really enjoy how racial powers are brought to the forefront of game-play with it.

Better Quest Objectives
Sounds of Skyrim
Interesting NPCs

His original opening post is now post #13 in this thread. It must have been replaced in the last day or two. To whichever moderator is responsible: thank you!

Director's Tools - I'm a huge screenshot enthusiast.

Mighty Magick - The magic system was borked.

Live Another Live - This alternate start mod is a godsend.

ENB, ENB, ENB!!

Right now my favorites are Superb R-L ENB, Deadly Dragons, and Levelers Tower.

The two most useful mods I have are Ring of Himalyan Hauling (lets you carry 10,000 pounds) because I got sick of running back and forth to sell everything and Book of Console which gives you two books for your inventory so you can easily look up any console commands in game.