Here's a list of my favorites, but I must admit that I am biased towards these simply because I am honored to have them in my collection.
1. Detective Comics #828 - When I met Simone for the first time at SDCC in 2007 I told him that this was my most favorite image of Batman. When he said that Sal had it available for purchase I had to get it. I love it and will probably never part with it.
http://www.comicartfans.com/GalleryPiece.asp?Piece=295145&GSub=397132. Giant-Size Thor#1 variant cover - I was going to commission Simone to do a Thor piece at SDCC 2009 but then this piece was there available. Soooooooo, another jem added to the collection.
http://www.comicartfans.com/GalleryPiece.asp?Piece=543533&GSub=397133. Astonishing X-Men Premier cover - My wife and I were floored when we saw this in person. You know you have a keeper when she tells you " If you're not going to buy it.....I WILL!"
http://www.comicartfans.com/GalleryPiece.asp?Piece=474465&GSub=39713As for my other favorite covers done by Simone (not owned by me, but I can wish..cant' I?

)-
Ultimate Captain America cover ( as seen in Simones gallery )
Detective Comics #820 ( also in the gallery )
Batman #654 ( awesome image of Batman and Robin with a Gotham City backdrop)