SW: IV, V, II, III, VI. I just showed my nearly 5 year old this order, and he loved them. He saw a good portion of I last year and it couldn't keep his interest.
Tolkien: My nieces and nephew were born in 99, 00, and 01, and they grew up on the movies. We get together every year for a marathon. I took the middle one to the Hobbit for her birthday and she loved it, but I think she prefers LotR.
ST: start where you feel it. I haven't gotten the kiddos into it yet, although my 5-er loved Enterprise when I watched it a couple years ago.