Simply put, what takes up the most room in your inventory/ what is the most annoying thing you keep having to manage constantly?
Mine are potions.

Crafting Stuff. Mostly Iron because hearthfire.

Random junk, mostly in the MISC category, followed by all the notes and journals you end up collecting along the way.

Potions and ingredients.

Mostly crafting items for me. Also for some reason I also pick up dragon bones and scales then don't sell them because I'll forget about them.

For some reason, even when using mods on pc and having a carrying capacity of 600+ I'll end up encumbered, and once I get rid of the things I think I don't need my capacity still sits around 400. I need to learn to manage my pack rat deal xp.

I like to hoard every enchanted item I find in a chest or lying around while questing. I usually sell all potions except for Healing ones, and only Misc items I keep is Gold, Lockpicks, and Azura's star. No scrolls, books, or notes, unless they're quest items. No keys, no food. Just enchanted stuff and the odd Dragonbone/Dragonscale

I voted for potions because I usually have a ton of those in my inventory. But the things I get the most of that really clutter my inventory are books, notes, and keys.

Potions.

These mount up weight-wise, each being 0.5 of a unit in weight.

Take a look in your inventory and see just how much dead weight you're carrying!

Bunch of enchanted apparel(like: Ring, gauntlets, necklaces), even stuff I might never use but I keep anyway out of fear that "I might need it one of these days".

My next character im just going to carry a town suit, fighting gear( 1 helmet, gauntlet, boots, torso) and just try to keep it minimalist

I just thought Id mention that Ive never had the Master trader perk on my previous characters, but my current mage character has it, and man is it easier to keep my inventory clear when all the merchants have 1750g and will buy anything from you.

Quest items ...
Neat idea in theory, incredibly annoying idea in practice.

Unique items! Until I finally bought a house and dumped them all in there.

Other: I have so much carrying capacity that I loot everything from slain enemies and containers. It's a force of habit; take everything, sell what I don't want, and keep the rest. I have thousands of soul gems on me at all times because of this, not to mention the 1.5 million gold I carry.

Leather, ores/ingots and the heavier alchemy ingredients like bonemeal or glow mushrooms.

Potions and misc items.

Potions. I wish they were arranged by effect. I get sick of scrolling constantly to try and figure out what i have. i wish all the restore health were together, restore stamina, you get my drift. It's hard to tell how many I have of each effect.

potions, i always get them cause they weigh very little and go for a high price, then i always forget to sell them.

Arrows. With mods there are like three dozen different kinds of them. Also, all kinds of stupid ass quest items.

Quest items! The damnable quest items! So many glitches with those, but after five different play-throughs I managed to take care of the ones that messed up. Then 1.9 came and I finally got to get rid of Gallus' damned journal.

I always have potions but they are not annoying they are useful and I always have a use for them.

There are some seriously strange weight balancing decisions in Skyrim compared to the previous games. Leather always has a weight of 2, even if you make leather out of a wolf pelt which only weighs 1. And then there's potions which always have a weight of 0.5, when in Oblivion they could be anything from 0.1 to 1.

I chose Soul Gems since at the moment I am using an enchanted one handed weapon (working on getting one handed to 100) that needs a lot of recharging, and it has soul capture so I need a lot of gems. I have a fair few of every type but black, the black ones I have already filled up, and there was a large amount of them.

I would have chosen other as well if we could have selected two, just for the all the quest junk I got stuck with. It might not weight anything, but they still all take up space I have to scroll through.

Dragon Bones/Scales.

Everything else I manage just fine. I typically don't raid for normal loot (no unenchanted armor/weapons below Scaled/Dwarven grade), I don't pick up food, and on a character that uses a lot of poisons he only carries one weapon (if any at all).