Overrated: Mantle. Great player, but he's not Babe Ruth. His cards don't deserve to be priced like he is the best player of his generation.
Underrated:
1929 R316 Kashins. They are beautiful, oversized and very rare. PSA has graded about
2,000 cards TOTAL for the entire set. The most graded card is Ruth with less than 200. Gehrig has been graded about 50 times. Both are beautiful images, with the Gehrig being the same Charles Conlon image (Conlon did all of the photography for the set I believe, choosing action shots for the most part) used for the '33 Goudeys.
Speaking of Conlon, I think sports photography in general is underrated. I was salivating at the offerings
RMY had last night in their auction.
Favorite: T206 Mathewson Dark Cap. I know its one of the most common T206 issues, but I think it is the most attractive.
Least favorite: As many others have said--cartoonish strip cards.