That depends on what you want to do, what type of play you prefer. Building settlements? Exploration? Weapons? Other kit? Different mods for each of those things, very different.
For example, if you're into settlement building then by far the most useful mod is Place Everywhere. If you're not into building settlements, that mod is useless to you. Ditto for Clipboad, Settlement Attacks Beyond and BS Defence. If you're into other kit, then Custom Combat Armor is an excellent mod. If you're not, then it's useless to you. Ditto for Eli's Armour Compendium. If you're into exploring, then mods like Lexington Interiors and Beantown Interiors would be great for you. If you aren't, they'll be useless to you. If you want bigger DLC-size extra areas mods and don't necessarily care much about them being lore-friendly, then mods like Fusion City Rising would be excellent for you. If not, they'll be useless to you. Etc, etc.
If you state what you want, I can give you my favourite mods for that thing. Most of the mods I use are for settlement building, but I have some mods for user interface, weapons, clothing, armour.
In general, I'd say these are very useful for all games:
Full Dialogue Interface:
https://www.nexusmods.com/fallout4/mods/1235
Fixes the deliberately broken dialogue interface so that each option shows what your character will say if you choose that option and not a summary that might or might not bear much resemblence to what they'll actually say. A wonderful mod for anyone who can read.
Scavver's Toolbox:
https://www.nexusmods.com/fallout4/mods/17507
Allows you to scrap all the junk you're carrying into its component parts in one go. Quest items will not be scrapped and will return to your inventory. Shipments will be converted into the right amount of the right material. Drop toolbox, open it, store junk, scrap junk - the materials and the toolbox are returned to your inventory. Works perfectly with everything apart from bags of cement, which aren't put in the toolbox when you select "store junk" but which can be added afterwards and will be correctly scrapped. I only use it in settlements because it seems silly to sit around in a hostile environment disassembling desk fans and suchlike, but it will work anywhere.
Screen Flicker Killer and no blur effect:
https://www.nexusmods.com/fallout4/mods/3191
Repairs the broken screen on your pipboy, so it no longer flickers, blurs and fills with scanlines. It makes it so much less annoying to use.
And these are at least quite useful for all games:
Automatically Lowered Weapons:
https://www.nexusmods.com/fallout4/mods/20093
It just lowers your weapon in 1st person when you're not in combat.
Disable Headbob:
https://www.nexusmods.com/fallout4/mods/17632
Disables headbob, unsurprisingly. The vanilla setting is bad enough to give some people motion sickness!
Healthier Brahmin:
https://www.nexusmods.com/fallout4/mods/16802
Makes brahmin look like animals that could thrive rather than sickly dying things that probably couldn't survive to adulthood let alone thrive.
Hey Thats Not Junk:
https://www.nexusmods.com/fallout4/mods/3149
Marks some junk items as not junk. You can choose which ones in the installer. Things that sell for enough caps to make selling them a better option. Things used in crafting. Stuff like that.
Highlighting Enemy and Corpses:
https://www.nexusmods.com/fallout4/mods/14484
I use it only for highlighting unlooted corpses (you can change settings with a holotape created by the mod). Useful for scavving after a fight.
Keep Commonwealth Radiant quests within the commonwealth:
https://www.nexusmods.com/fallout4/mods/19967
Does what it says - no more being sent to Nuka World or Far Harbor for a commonwealth radiant quest.
SarcasticDragon's Snarky Loading Screens:
https://www.nexusmods.com/fallout4/mods/25726
Edits the loading screen messages (VDSG catalogue entries) for added swearing and sarcasm. Amusing rather than useful.
I'd also suggest looking for a mod that disables Minuteman radiant quests because they are endless. So much so that they're still a meme. You mention that you've played FO4 before, so you're probably familiar with Preston Garvey and his endless stream of settlements that need your help. I can't help you there as I'm using the first such mod (Shut Up Preston) and that's not recommended any more because it can break your game. It predates the creation kit and the mod author was working without required details. The mod works, but it can break the minuteman questline and
will break your game if you uninstall it. I've seen some others that don't, but I know nothing about them.