St Albans, Georgetown Prep and St Johns have had the strongest baseball programs in the private school arena.
Maret has had some good years as well.
Last year, Dematha, Prep and Maret won their respective conferences. Here is a list of all the teams and their standings related to their conferences. Teams typically do not play teams in other conferences except for the DC schools. There is a DC tournament.
http://www.thesportsfannetwork.com/articles/6262-baseball-standings-wcac-iac-mac/
Baseball is not as popular in the IAC as Lacrosse. Lacrosse is king and baseball does seems to get short thrift.
The numbers of kids playing baseball have really gone down so keeping a quality program going at all these schools is difficult.