My passion these days is animation cels from shows I watched as a kid, so I've binged on quite a number of cel purchases in the last month. While I wouldn't go so far as to say they were my ultimate holy grails in this specific hobby, they are certainly the top of my crown jewel list right now.
Spider-man cel from the '60s show. It's a great image from the intro theme song, so it's especially difficult to find:
One of my favourite movies of all time is Transformers: The Movie from 1986. My ultimate holy grail is a nice Optimus Prime cel, but these cels from the movie aren't too far behind:
All are sellers' scans/pics.