The NRaas mods that @igazor recommended for taking care of glitches are very important, although of those I only use Overwatch, Register and Traveler. I would not play Sims 3 without those.
Also must haves for me are Mastercontroller and its add-on mods and Tagger. They make my job as the "sim god" so much easier.
Another important NRaas mod is StoryProgression and its add-on mods. It determines the composition of your town and controls the "stories" that the townies operate under. It is complicated setting up and can cause lag on under-powered systems so I would try the other mods first.