Yea, the only modern things I've read by him were the Just Imagine books and I can't say much for them really. I've read some of his original X-Men and Avengers stories, along with some Spider-Man things as well, but I can't think of anything to recommend myself.
I liked a lot of his Spidey and X-Men stuff but not too much else. Well, I did love the intro to Galactus, that was nifty. I'm sure if you do a search for "stan lee's best work" you'll find some stuff somewhere.
Yeah, I know he stayed on FF longer than any of the other titles, so I would assume there's something good there, but I've just never gotten into the FF.