WGA’s Angle On series features a video interview of several screenwriters at Sundance, discussing the constraints and advantages of writing for indie film, and some of the best advice they’ve gotten in their careers.
I’ve done lots of poking around and by and large the people who should know these things say that the Nicholl Fellowship is the gold standard. It’s run by the Oscars (which is also the source of one of my favorite reputable screenplay format documents – PDF) and it’s got the best track record for providing a real boost to the careers of folks who make into final rounds, not to mention winners.
I’ve heard some good things about Bluecat — they offer written analysis for every script, which may just be worth the price of submission.
One other consideration is the genre contest… a good friend won the H.P. Lovecraft Film Festival and CthulhuThon Screenplay Contest 2008 (whew!) and while the niche is very specific, the fanbase of said niche is dedicated. Jeff Palmer’s “The Sleeping Deep” screenplay has gained some well deserved attention (and it’s little known that his winning version was a first draft) and now he’s on to shooting a trailer to generate more interest in mounting a production. So if your screenplay fits a niche genre, the benefits of a lesser-known niche contest may be worth considering.
Beyond that, tread carefully. While the contests may be legit (and no, they aren’t all legit), the point is that you want your win/place/show to add perceived value or legitimacy to your script. If nobody’s heard of the contest, how’s that gonna help?