My very first mod was for Sims 2 and it was so that I could do an entirely medieval world with all the townies in medieval clothing, whatever the weather, carriages instead of taxis and everything you could think of to make the world truly authentic. It took me weeks to get it all set up the way that I wanted.
I still have the save but am on a different machine now and would need to reset everything manually again. I summoned every townie to a lot with the clothing rack mod and manually changed all their outfits one by one. The thought of doing that all over again is too daunting for me to try it, though every now and then I do briefly consider giving it another shot.
I don't remember my first mods in Sims 3, but I had way more in that than any of the other incarnations.
I only recently installed my first two mods in Sims 4. MCCC (installed a few weeks ago to fix a relationship stuck at acquaintance issue since the EA code to fix it was removed from the game) and a little mod to sort out the recent can't build the rocket ship issue because of some issue relating to the festivals. I would probably have waited for the EA fix for that, which presumably will come at some point, but one of my sims needed to be working on it to progress in her career.