Personally, I didn't really enjoy Torchlight 2 all that much until I played it with the Synergies Mod. In addition to filling out the entire game with more unique content throughout and after the main campaign, it rebalances the core difficulty levels, which I found to be out of whack. I'm no action-RPG veteran, but Normal was WAY too easy and Hard was too tough. With Synergies, Normal is challenging when it should be without being super tough all the time.
Sadly, Synergies on Workshop is confusingly implemented and requires several different subscriptions to be put in the correct order. The answer to which to put in what order should be on the official Synergies website.
Synergies is absolutely worth the trouble though.
Aside from that, the "Blanks Landmarks" and "Extra Chunky" mods are recommended because they both add more variety to dungeons and landscapes. Both mods were developed by people who worked on the original game.
