It would have to be the Higgs particle. Hands down.
No doubt, #higgsy was a big one. Who knows though, curiosity could have discovered some Alien microbes.
That hasn't been discovered they assumed it existed then looked for evidence to fit their assumptions. "The Higgs boson or Higgs particle is an elementary particle in the Standard Model of particle physics. It is the last unobserved particle of that model and has been predicted to exist since the 1960s. It may have been detected at the Large Hadron Collider in 2012. Proving its existence would settle questions about the existence of the extremely significant[6] Higgs field—the simplest[7] of several proposed causes for electroweak symmetry breaking and the means by which elementary particles acquire mass.[Note 3]" - Wikipedia I am just skeptical after the debacle of them claiming electrons traveled faster than light.
Well, there is a very high probability (>99%) that the LHC detected a particle with a mass consistent with the Higgs theory. I think it's important to realize that there isn't any 'them' here. The scientists that thought they detected faster than c electrons are not the same folk. Also those scientist and most others were very skeptical of the electron results. They announced it to get feedback on where they might have made a mistake. The LHC data could be wrong, but I think the chance is extremely low considering all those that are scrutinizing it.