the one who/that got away
Someone or something that one regrets having lost. You're never going to be happy in your love life if you spend the rest of your days pining over the one who got away. He still talks about that failed deal as the one that got away.
one that got away
Fig. the big fish that got away, especially as the subject of a fisherman's story. The one that got away is always bigger than the one that got caught.
the one that got away
something desirable that has eluded capture. This phrase comes from the angler's traditional way of relating the story of a large fish that has managed to escape after almost being caught: ‘you should have seen the one that got away’.