These are the most interesting challenge topics, I think. 16 thru 23 cover a diverse mixture of players, card types, eras, and personal favorites.
16. A card of a player whom you appreciate (respect as a player) but don't like.
Honorable Mentions (most of which I'm not big on as players either) - Dustin Pedroia, Ryan Braun, A-Rod (just weird), Mark Teixeira, AJ Pierzinski.
17. A card from the first set you put together by hand.
This is really the same as the #3 challenge, as I've never actually tried to complete a set and FAILED. Some of them just stay on your want lists longer than others.
A more interesting twist on this one would be to show which card was the last one you needed to finish the set (see my #22 below). A lot of people would be posting big stars or high numbers for most vintage sets, but it would be cool to see that elusive Micah Bowie or Jack Billingham that someone had to chase. I've already posted my last card to complete the 1979 Football set, and I can't really remember which 1980 baseball was the last one (I know it wasn't Rickey), so here is just another typical 1980 Topps card.
18. A card of a player who became manager of your favorite team.
Frank became the first manager the Nationals ever had. He also led the Orioles for a few years in the early 90's. So he covers both my local teams.
Honorable Mentions - Dusty Baker (a PC of mine already),
19. A favorite card from a country other than the United States.
Livan Hernandez XRC from the 1994 Cuban set. Figured it was printed in Cuba too. Nope! Back says "Printed In Canada". I was really trying to avoid the default O-Pee-Chee answer!
If I could have used a hockey card, it would have been a Sergei Fedorov from Russia.
20. Your favorite parallel card based on the parallel, not the player.
When I think of parallels, anything made of acetate (Topps Clear), Blue Chrome, or anything by Pacific in the 90's would make my short list. But I ultimately had to pick these framed blue parallels from 2012 Gypsy Queen. They're just so retro-elegant, and the frames give them depth that makes even the most cliche picture into a modest work of art. The GQ logo is a bit in your face, but overall these are my winner.
21. A card of a rookie you thought you were "investing" in.
I am so NOT into rookie prospecting, so here's a bit of a twist on this theme.
I once found a shop while traveling and they didn't have a whole lot, but there was one box of really nice vintage from 1962, '63, and '64 for 25¢ each. I picked out a big stack of singles (which became my starter lots for those sets - '64 is now officially active) and brought them to the counter. The kid at the register looked at my pile and said "That looks to be about 100 cards." and rang me up. I paid and left and almost sprinted down the street because I had actually pulled about 330 singles. When I saw these two in the piles, I thought I had struck gold at an even higher level! But alas, these are not actually the rookie cards of Gaylord Perry and Tony Oliva. Oh well, still a bargain!
22. A card of a common player that always seemed to elude you.
There are more elusive cards for me in other sports. Still looking for one single 2008-09 OPC hockey single to finish both series and all the inserts, and a couple quarterback rookie cards to kill football sets.
The next section will deal with oddball cards from all the decades.