Grail is such and ugly word (in my best Inspector Kemp voice). There is no one piece that if I got would end my comic art collecting (the addiction is just too strong). If you are talking about a piece that had a special significance to me it would be a few, a page from Silver Surfer #1, one of the pages where he is transformed by Galactus. (I did get John to recreate it for me but is isnt the same).
Another would be Daredevil #47 "Brother, Take My Hand" any number of pages from that story. (only ever seen the cover) A favorite of mine growing up.
Last Uncanny X-Men #153 "Kitty's Fairy Tale", another favorite story. I did get the DPS which shows all the characters and the opening swashbuckling scene. I could say I achieved a "grail" but then again I am never comfortable with that word. Lets just say the DPS, I have wanted for years, and never thought I would own. Now that I do its one of the gems of my collection. And no, I won't take stupid money (or any other kind of money for it).