few of mine;
-lost a 98% Kyogre on its first day, when it was very difficult to catch (they subsequently made it easier.) I didn't raid many Kyogres and ultimately never saw another one in the 90's.
-Lost a 98% Moltres when it was first out. Never seen one above 91% before or since, from a total of 50 Moltres raids. I did however catch my friend a 98% one on community day. Lucky him. I traded a pre september 2016 pokemon for a shiny (guaranteed lucky) but that came out at just 14-12-12. Moltres doesn't seem to like me.
-Lost a 98% Lugia on the first day of Lugias. I caught a 95% one about eight months later. The only one i got in the 90's.
-Lost a Mewtwo from an ex raid last October. This Mewtwo did not have great IVs but it was my first ex raid, at a time when few people had been given an ex pass. I sacrificed a weekend away for this, while the group booted me that same day. (long story.) Fortunately i have since moved away to a different town. I had to wait 7 weeks for my next mewtwo raid, when i caught a dismal 10-12-10 Mewtwo.
-A 95% Magikarp ran from me.
-A Golem ran from me in the games early days, owing to the very buggy nature of the game back then. Took me weeks to get enough candy to evolve one.
I have never allowed any shiny Pokemon to run from me but i'm sure this has happened to some people.
What pokemon have you lost that were valuable and made you rage?