Lots of good examples of 'one only' Omegas. But to me the key model that has defined Omega over the last five decades is the Speedmaster. I've had at least one Speedmaster in my collection since 1972, but the one that really touches all bases is fairly new, the Apollo 8 Dark Side of the Moon. The Apollo 8 mission was the first crewed flight of the Saturn V rocket, the first time men had escaped earth's gravity, the first visit to the moon, and Earthrise. People finally understood that all that 'going to the moon' talk was real, it was happening!
The Apollo 8 watch is a modern size made out of modern material, but still has a movement that has a lot of the original DNA brought to life with a very unique finish representing the light and dark sides of the moon. And it is thinner to boot. A very easy to wear watch and nice to look at during the day. If I could only have one Omega I'd take the Apollo 8.