While doing research, scientists and engineers read (at various levels of depth) many papers. Every now and then, you come across one that is really great, yet somehow doesn't seem to have received the attention or appreciation it deserves. I'll pick one here, and hopefully some readers will put their examples in the comments.
One that I like a lot is this paper from Wilson Ho's group at UC Irvine. Here the authors use a scanning tunneling microscope, and demonstrate that when the tunneling current-voltage characteristic, they get rectification of microwaves. That is, when microwaves are applied to the tip-sample junction, the result is a dc current proportional to the square of the microwave amplitude and to the nonlinearity (second derivative of I with respect to V) of the tunnel junction. It's a clean, elegant experiment, with quantitatively accurate comparison of experiment and a simple classical theory - very very nice, and really underappreciated in my view.
Any suggestions of others?