Well I lost my beautifully beat DX Roc earlier this year (it was one of the waxy runs so it only took 2 years to get beat). I've since replaced it with GL Fuses. They're not the same at all, but with some hyzer I can get similar lines out of them. I threw that DX Roc on a very risky shot that I knew was a risky shot, but it was also by far the best disc in my bag for it so I threw it anyway (and paid for it).
Currently, I'd probably be saddest about losing my nicely beat in patent numbers Star Destroyer. I wouldn't necessarily be saddest because it would be the HARDEST to replace (there are loads of people buying/selling old Destroyers all the time), but it probably would be the most expensive to replace with the same mold. I've bagged that disc for a long time now though, so it also has some sentimental value I suppose. I'd actually probably just replace it with one of my FR Sirius Scorpiuses. Again, not exactly the same flight, but similar enough.
If I were so attached to a disc that I couldn't stand losing it at all, then I would just retire it. I retired my very first golf disc a few years ago (Christian Sandstrom Star Valkyrie). I'll throw my old Destroyer on mildly risky shots although probably not to carry water. That's what DX Thunderbirds are for.