I just booked a quick trip to DC to see the Cubs there Labor Day weekend and that will make 24 total and 19 active parks for me. I have Seattle, Anaheim, San Diego, Arizona, Mets, Yankees, Toronto, Philly, Atlanta, TB, and Miami left.
Wrigley, SF, Baltimore, Great American, and PNC are probably my current top 5 in no particular order. I went to Arlington and Houston last month and while I appreciate the indoor park so you're not sitting in 100 degree heat most games, indoor baseball kinda sucks.