Wow, that's a hard one to answer since it requires me to f-o-c-u-s, but here goes:
Exhibit and other arcade cards: Lots of answers here. Interesting subject matter, archaic technology, attractive formats, relatively low prices, a range of easy and extremely challenging issues.
Ray Miller [boxer]: cousin. Nuf ced.
Joe Louis [Boxer]: Ten years before Jackie Robinson, there was The Brown Bomber. Lots of things to collect, wide range of items from common to very, very rare.
Boxing cards: Nearly all of them, from all over the world. The sport is my favorite, the cards are amazing.
T206: Got my 1st at age 12; can't imagine not having some.
1954 Topps: My favorite modern set. I've assembled and disassembled it multiple times over the years.
Zeenut HOFers/popular players: Minor league pre-rookies; how cool is that?
Lefty O'Doul: Great player, fascinating career. Getting one of each of his cards also gives you a very broad type card collection from the late 1920s to early 1930s.
Jim Jeffries [boxer]: Burbank's leading citizen from the early days of the 20th century. Had a farm right near my office where he held boxing and wrestling matches, amateurs. A true sportsman.
Los Angeles-related boxing photos and memorabilia: Everyone knows NYC's boxing history but LA had a heck of a thriving fight scene too. From Jeffries Barn in Burbank to Doyle's Arena in Vernon to the Olympic Auditorium downtown and in lots of smaller venues. Dovetails nicely with my interest in Hollywood history and movies too, as lots of movie executives and actors were into boxing.
1930s photo of the boxing and Hollywood glitterati at a fight in Los Angeles. Standing L to R: Dan Toby, Bud Taylor (HOF), Jackie Fields (HOF), George Raft (actor), Ceferino Garcia, Ace Hudkins, Pat O'Brien (actor), James J. Braddock (HOF), unk, unk, Jack Root (HOF), Al McCoy, Tommy Ryan, Unk. Kneeling L to R: Fidel La Barba, Jimmy McLarnin (HOF), Joe Rivers, Willie Ritchie, Gorilla Jones (HOF), Billy Coe.
Jewish athletes: a small collection, needless to say [J/K]. Shown here is the great Benny Leonard, king of the lightweights:
There's more I'm sure but this is all I can think of right now.