TBH, I get a lot that I could from watching it once, but for the **** of it, I'd probably watch a series again. W is something I'd like to do because of A-to-Z and Eternal (and A-to-Z was the first movie I rewatched: first release and director's cut). I don't like to rewatch because I'd know what happens next, but there are times when I would do it anyway.
If I ever finish Agito, I'd probably rewatch Agito. Kuuga, not so much; when MCS was doing it, I almost dropped because I was SO bored. The action was awesome, but I always felt it was a bit repetitive. I won't deny that it's one of the best Heisei series, but not one I'd want to rewatch.
Den-O, I think I'd rewatch because i need to figure **** out...