As of right now, I have 180 active mods on Skyrim and here are my top 10. Note that These links are all from Nexus mods and are for the Special Edition but I believe that they are all available on the standard one as well. Also note that this list will focus on content mods so bug fixs and utillites such as the Unofficial Patch, SkyUI, and Amazing Follower Tweeks though recomended (and in some cases required) arn't included.
Sjel Blad Castle
A MASSIVE customisable player home full of NPC's that you can hire scavenge in the world for Alchmical ingredints, ores, weapons armor, meat, pelts.. you name it! There are also traders, followers, animal compantions and a teleport system to each of hold capitalsd and solsteim. It would take to long to explain it all and I would conmsider this the ultimite player home mod if weren't for two problems. Numbner one is that some features are incomplete (and may never be) and that one of those features is the childrens wing so it not Hearthfire compatible.
Lakeview Extended
With that in mind If Hearthfire is a concern, I recommend this one. It adds to the original Hearthfire player home with new buildings, features, trader NPC's, followers but without de-railing or restricting the original Hearthfire experience.
DeserterX Armours (Some are NSFW and require a body mod like UNP or BBE)
These are some of the coolest, beautifully crafted and in many cases, sizzling Armor mods I have seen and the guy is still putting them out. They certainly won't be for everyone but I love these designs.
Immesrive College of Winterhold
If you ever thought that the College of Winterhold was a little drab for a place dedicated to the study of magics this will certainly fix that. It adds a bunch of cosmetic changes like furniture, wall coverings and artifacts. Also new botanical gardens, alterations to AI so that the other students actually participate in classes practicing their spells and even a new "The Midden” Questline where you get to explore the ruins of the fallen Winterhold. It makes the College feel so much more alive.
Sofia - The Funny Voiced Follower
Sofia is a follower fully voiced custom follower who you find passed out in the stables outside of Whiterun. She tags along with making off-color comments and jokes about you, the game, NPC's, quests often breaking the 4th wall and it is hilarious! You can even give her alcohol and she wanders around blind drunk! Well worth at least one playthrough with her... until she totally gets on your nerves!
M'rissi's Tails of Troubles
Another custom voiced follower but this one quite different. It features a full questline with multiple choices and you try to uncover M'rissi's past and keep her safe from the Aldmeri agents that are pursuing her. It's a gripping and at times very dark adventure.
The Kids are Alright (With Hearthfire Multiple Adoptions)
Unfortunately, as I have just found out this mod is no longer available on Nexus because some asshats where reuploading lewd versions of it so the author (understandably) set it to hidden. The child NPC’s in the original game where a bit of an afterthought and all looked and dressed pretty much the same. This was a complete overhaul of the children in the game giving each of them their own unique look as well as adding new clothing sets (including armor), a bunch of other adoptable and even hostile NPCs to the game. You could even adopt vampires and undead skeletons if you wanted to! Shame it's been taken down, guess I'll have to keep a safe back up of the archive file now.
Helgen Reborn
This is an extensive lore-friendly quest mod where you, of course, rebuild Helgen, the town that was destroyed at the beginning of the game. Fully voiced NPC's (including one friggin hilarious Arnold Schwarzenegger impression), new armor, followers, merchants, and a player home to top off a great story.
The Forgotten City
This is a mod that is so amazing and got so many accolades that not only did the guy that made it got a job in the game industry full time as a developer, but the mod is getting expanded into its own release as a full stand-alone title! I don't want to give away to much but it starts with a letter from a woman who has lost someone in a forgotten Dwarven ruin that you will have to explore to find a way out. The design of the quest is simply brilliant where you will have to think outside the box to solve the mystery of the place and get out alive.
Legacy of the Dragonborn
Quite simply, The One Mod to Rule them all!
Legacy of the Dragon is a gigantic expansion to the original game that had been added to, patched, and updated for years. Lore friendly with Dozens of new fully voiced NPC's and a New Guild. It' got Multiple questlines, dungeons and player homes with every utility you can possibly imagine. Then there is the Legacy of the Dragonborn Museum itself, a huge complex with custom displays for pretty much every single unique weapon, armor item, book, and more in the game and DLC’s. Not to mention it adds new and much-improved skins for them as well as adding hundreds of unique new weapons and items of its own. Hell if you add the right patches you even get custom displays to form unique items found in other mods like The Forgotten City and Helgen Reborn.
It's INSANE how much content is in this thing! I reckon it adds at least another 80 hours to the game and none of it even feels like a filler! If you were to install just one mod to Skyrim, make it this one and any patches that you need.