QOL:
Smooth Save[valheim.thunderstore.io]: trade off higher RAM usage on world save for much lower likelihood of freeze. Doesn't fix the post Mistlands ~10min freezes, but the upcoming patch is supposed to fix that.
AAA Crafting[valheim.thunderstore.io]: Adds option to enter a number of thing to craft, rather than pressing craft over and over. Also adds search, and (optional, by default) paginates the craftable list.
Contents Within[valheim.thunderstore.io]: Uses the games own UI to show contents of containers on hover, I find this useful and preferable to signs everywhere.
Spear Fishing[valheim.thunderstore.io]: shoot fish with bows, spears, etc. Useful if you hate fishing and/or want access to fish prior to finding the trader.
Azu Area Repair[valheim.thunderstore.io]: Repair building pieces within an area. Still uses the stamina that would have been required, so can completely drain stam and take a few whacks to fix everything. Especially useful if you build with hidden structural pieces and your building takes AoE or rain damage.
Target Portal[valheim.thunderstore.io]: "Lets you target any portal, instead of having to connect two portals via a tag."
Instantly Destroy Boats And Carts[valheim.thunderstore.io]: Allows build hammer right click destroy for boats and carts. Optionally (by default) prevents destruction if occupied by player or while containing inventory.
Farming[valheim.thunderstore.io]: Adds a farming skill, with various and configurable QoL benefits at configurable skill levels.
(A) Deez Misty Balls[valheim.thunderstore.io] or (B) Mist Be Gone[valheim.thunderstore.io]: (A) Adjust the wisp clearing radius, or (B) remove mist entirely.
I'm FRIENDLY DAMMIT[valheim.thunderstore.io]: Make ballistas not shoot you. (I haven't personally used this one, but I've used some of their other mods and I've seen others mention this one).

Game changes:
Azu Skill Tweaks[valheim.thunderstore.io]: increase or decrease skill gain, as well as death penalty.
Death Penalty[valheim.thunderstore.io]: "Tune the death penalty skill loss percentage. Modify the duration of the "No Skill Loss" & "Corpse Run" buffs." Should be compatible with the above, this sets percent while the above adds a modifier to the percent. While I have both installed I haven't died yet to test this.
Custom Raids[valheim.thunderstore.io]: Lets you completely change the raid system, from small changes like disabling completely or uping/lowering the percent change and check timers, to creating your own raids.
Comfort Tweaks[valheim.thunderstore.io]: Adjust the comfort system. Could be just QoL, but you could make it worse if you wanted to - or cheaty.

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I'm sure there's a lot more mods I've used, and more still that I'd try, this is just what's currently on my radar that I've actually used.