The "Wow!" Signal - unexplained SETI radio observatory signal. Either the closest we've come to scientifically detecting aliens or a bizarre case of interference.
The "Oh My God" Particle - An ulta-high-energy cosmic ray particle, believed to be a proton traveling at 99.99999999999999999999951% the speed of light. Fastest thing ever observed.
The original "Eureka!" Moment - Have to include the famous historical incident where Archimedes purportedly discovered the principle of water displacement by volume. Also possibly first documented incident of streaking. ;)
