I never played it but the Thomas the Train mod for skyrim is hilarious.
GB Mario 2 in full color
Tale of Two Wastelands for Fallout 3 and New Vegas.
Gregtech: New Horizons
Technically a modpack rather than a mod though. I love grinding through technological advancements.
promods for ETS 2 and ATS 2
Mekanism, hands down.
Followed VERY closely by Fallout 2 Restoration Project.
I can’t believe I blanked on them in my own reply, but to piggyback off your Restoration Project nomination: the Talking Heads Addon and Talking Heads Actually Talk Mod for Fallout 2 Restoration Project (updated) are both absolutely insane, and breath some new life into the game while seamlessly slotting in next to the existing art and voice acting.
I love mekanism, often run it as my only mod
Sven-Coop for Half-Life 1.
I’ve played, almost daily, since the mod was released in January 1999. Over a quarter century. I’ve got admin in a number of servers, so a lot of days I’m just hopping on briefly to make sure the servers are running smoothly and no one is trolling. New maps are still being regularly released, many by people who have been mapping for it for a solid 15-20 years.
No idea of total hours. It wasn’t added to Steam until 2016, but I have multiple thousands of hours logged since then. I’ve got to be over 10-12k hours since the beginning. Easily.
Everything else in my life has shifted. Sven-Coop is my rock.
Rimworld multiplayer.
I really want to check that out, but it is incompatible with my favorite mod: Save Our Ship 2. I refuse to give up SOS2 just to play with another human.
Its pretty smooth but I’ve had a lot of desync issues because I have a long list of mods I try to play with.
They do PvP on the mod. Its pretty cool you have 4 teams and each player controls 1 pawn and the fight for objectives and capture and craft their way to victory.
I couldn’t pick a single one, but I got endless joy from all the Quake mods back in the day. Not just level designs and sprite replacement, but complete weapon changes that made everything different, Quake Kart, Cujo, etc. It seemed the sky was the limit.
I don’t know if time has been kind or not, but when I discovered Fall from Heaven II for Civilization VI I never launched the base game again.
I think I haven’t played a single skirmish round of Company of Heroes 1 without the Blitzkrieg mod in the past 15 years.
Terraria - Calamity.
Expands the game to 2.5x the content.
Absolutely amazing, would recommend buying Terraria just to play it.
Garry’s Mod
It’s hard to think of any one mod that got remotely close to changing a game the way GMod did for HL2/CS:S/source engine titles. I spent thousands of hours in GMod as a kid, it added infinite teplaya ility to the HL2 campaign, forums like Facepunch and PHWOnline became my second home. There was a ton of content to be loaded from there and FPSBanana, the thriving webcomic scene was truly special.
Better Than Wolves it’s a minecraft overhaul that was started by the veteran game designer FlowerChild who was pissed at Mojang that they are adding useless features that don’t improve gameplay loop (wolves being the breaking point). So he spent many years making Minecraft challenging survival and interesting tech game.
While I haven’t beaten the dragon once in that game (not many people have) it still gave me hundreds of hours of entertainment.
Gigastructural Engineering for Stellaris… the megastructures included in the game and DLC are great, but they lack a certain insanity. Plus the mod allows you to terraform any planet (even gas giants).
