If it’s by mass, then photons (particles of light) have no mass and would be counted as the smallest.
If you mean by size, then I’m not sure. I *think* that one of the types of neutrino is perhaps the smallest.
I’m not very sure of my ground here. Particle physics was not one of my favourite topics, in part because the current theories on the structure of matter seem a bit ‘cobbled together’ to me.
The “standard model”, which is our current view of what matter is made of, has got problems. Part of me is hoping that the Large Hadron Collider won’t find the Higgs Boson because that will force all the physicists to go back to the drawing board to come up with a better theory!