I'm at most a casual watcher (and haven't watched in quite awhile). My recommendation is to basically catch random episodes as they're virtually standalone. Watch the first two episodes (not sure if it's technically a two parter, but they go together). Then you can probably get away with catching whatever episodes you want. I'm sure someone who is a bigger fan can tell you especially good episodes or especially quality seasons. However, the first few seasons were good because the show was fresh.
CSI Miami is corny as hell. I don't think it adds much besides Horatio Sunglasses Memes
. CSI NY strikes me as a more depressing, less funny CSI Miami, but I've never really watched it.
Anyway, those are my thoughts.