Overrated - General: All Jasson Dominguez cards (Never played in MLB; cards go for more than the rookie cards of the top .001% of all players in MLB history). Here are his Baseball America Prospect rankings:
Pre-2020 #38
Pre-2021 #33
Pre-2022 #87
His RC sold for $475k earlier this year.

Overrated - Vintage: 1933 Goudey Ruths. There are 4000 of them in the PSA Pop Report.

Underrated - 1950 Bowman #22 Jackie Robinson. Only 1112 in the PSA Pop Report. Beautiful card. Compare with 1949 Bowman, where Jackie's card has a higher pop count by 20%.

Favorite - Home Run Baker 1912 "Series Of Champions" (T227) Baker was a great player, but some remember him for simply discovering Jimmie Foxx.

Least Favourite - Agree with everyone who mentioned strip cards. These are comic strips, not cards, and the drawings are so poor you need the name listed to know what player they claim to represent.

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.