Since I only own one other piece, might as well go all in and post it :)

Bob Layton's Iron Man corner box art

CAF: http://www.comicartfans.com/gallerypiece.asp?piece=1350584

After seeing your two killer Byrne examples, I have to ask what would your Byrne Grail piece be? 

Mine would be the DPS from X-Men 115 Wolverine vs Sauron. 

Hi Steve!  Thanks for the kind words and for posting the Bullpen image!

Yes, I agree that Wolverine vs Sauron DPS is probably one of the most dramatic and explosive pieces out of Byrne's entire run on X-Men. The details are incredible especially considering how huge each of the character images are!  And the one of Wolverine is definitely one of the greatest of all time from any book!

The Bullpen piece has always been tops on my personal wish list.  I like pieces that are quintessential examples of the subject.  I could never have a chance at acquiring the first appearance of Iron Man, but I feel the corner box piece is like having a "defining" piece of the character.  When I look at it, to me, it is Iron Man.  I feel the same about my FF corner art.

That being said, I really wish I could've acquired the X-men corner box art that was available not long ago.   I tried my best to send strong offers to the seller, but it didn't work out.  I would've really liked it to round out my John Byrne art collection.

Those X-Men heads would have been a spectacular addition to any collection!

This was the image posted to announce the piece was available a while back, not sure if it's has been sold or not.