Rapido coupler. Looks awful, but it is one of the most reliable couplers ever designed.
The Rapido coupler is the standard for N-scale trains, and has been for some 30 years. Rapidos are easy to couple together (even if they take some fiddling to uncouple), and they hold together reliably over a wide range of conditions, pushing and pulling, up inclines and at funny angles. But they are ugly, and they are huge. Most modelers in North America opt for the Micro-Trains Magne-Matic coupler, which not only looks better, but offers semi-automatic uncoupling which uses track-mounted magnets instead of hand-held toothpicks to work. (The N-Scale Division has a great history of n-gauge couplers.)