From the ones I own would probably be the Railmaster: Money aside, probably be the 40th anniversary Apollo 11 in platinum:
I only have one Omega and looking at possibly an Aqua Terra or Speedmaster Racing in the future. But the Planet ocean 8900 is the perfect (travel) watch for pretty much anything for me. On land, in the air and in the water. Looks great on bracelet and fabric/leather/rubber strap. So that would be my choice for sure.
If you could only have one it would definitely have to be a dive watch considering they look great whether your wearing jeans or a suit. For me, I'd take the simpler 300m over the planet ocean.
If only one, I sure like the Constellation “pie pan” Stainless steel is versatile and can be dressed up with a alligator black strap or more casual with with other types of straps or a “beads of rice” bracelet.
If I'd have to keep one watch from my watches, then it would be 1958 2872 cal 320 chronograph. For daily, it's not a very reasonable choice of course, but just look at it! If I could have something else from Omega, then Ed White or any other cal 321 Speedy would be the ultimate watch for me. If I could only have one watch from any maker - Zenith El Primero A3818 "Cover Girl".
It would have to be my trusty Planet Ocean. My travel companion on almost every trip I make. I sometimes regret not wearing it more when I’m not traveling. Now when the steel AT Worldtimer is out, I might just change my mind...