The Law & Order Cast Spreadsheet has been my little baby for a long time now - after finding an embryonic version of it online way back when.
Over its eighteen seasons, the original series has had twenty-six different cast members. Eleven in the last four seasons alone - including two new recruits for the coming year: Jeremy Sisto as Detective Cyrus Lupo to partner Detective Green and Linus Roache to play EADA Michael Cutter, taking Sam Waterson's position in the series paradigm... while Waterston's Jack McCoy fills the role of District Attorney.
Law & Order is the longest running series still on air in the Tommy Westphall Universe. The original twenty-six year run of Doctor Who takes the longest-running drama title... at least until we fold in those recent daytime soap opera additions.