For me, it's sound primarily. Loud noise alone, regardless of the type of sound, can genuinely cause me so much distress. In fact, there are times where it only takes one instance of something like a loud car or motorbike, a balloon bursting or someone shouting (regardless of whether it's even at me or if I'm just near somone shouting loudly) to completely ruin my day. I literally never leave home without earbuds/earphones and my phone, so I can play music or sounds to reduce the risk of this happening; monotonous sounds like white/pink/brown noise and recordings of rain are often what I listen to when I get overstimulated, they can often calm me down or at least reduce the panic and stress that comes with sensory overload.
I'd say touch is my second most sensitive, as there are certain textures that I will actively avoid touching because they distress me, such as surfaces like old brick walls that have a chalky or dusty residue.
Also, I'm very specific with a particular aspect of smell, as I will outright not eat something if it smells a certain way (regardless of whether the taste of the food even resembles the smell). When I was a child I used to have to sniff pieces of bread before a parent made a sandwich for me for schooll, as an example.