It depends on what kind of games you like, but in the past few years ones that I've enjoyed that have great mods are: Rimworld, Slay the Spire, and Factorio.

A good way to find games that have mods is to look at the "Steam workshop" page on a steam game listing. Here's the listing sorted by games that have a "Steam workshop" (which is kind of steam's interface for managing mods): https://store.steampowered.com/search/?category2=30