Transmon
The superconducting qubit design that became the industry standard — insensitive to charge noise, and therefore long-lived enough to be useful.
The design was presented by a group at Yale in 2007. The problem with earlier charge qubits was that the slightest fluctuation in surrounding charge wiped out the state within nanoseconds. The fix was to add a large capacitor in parallel, sharply damping that sensitivity. The price is that the energy levels move closer together and become harder to address individually — a trade-off that paid off by a wide margin.
Coherence times went from nanoseconds to hundreds of microseconds. IBM, Google and most startups build on transmons, which makes it the field's workhorse even though it is not the theoretically most elegant solution.