I was asked this in another community. This was my answer, which I think sums it up nicely!
"In pure monetary terms it's the ugly piece of paper in the first pic. British Guiana 1856 1c black on magenta. One copy is known. The catalogue value isn't shown in monetary terms, but is indicated with the infinity sign ∞, the only stamp listed thus.
It's valued at around $10,000,000
However, if you ask a hundred collectors what their Holy Grail is, you'll get a hundred different answers!
Mine is in the second pic. I need it to complete a set. I had a copy, but traded it for a room full of stamps (yes, really, we're an odd breed), and I only need around £800 for a new one. Once I get it, I'll have a new Holy Grail, and thus the quest continues!"