Over the years I have settled on the following "standard" meta data for movies that I populate in Movie Collector. Not too much, not too little, with a nice mix of descriptions and reviews.
IMDB - brief 1 or 2 sentence description, year, cast, crew, rating
Rovi (AMG) - nice professional description, nice professional review, rating, also good for checking IMDB cast and crew
Amazon - professional review
DVD Empire - best high res front and back covers, distributor's product description
Wikipedia - optional extra information for special movies
I am interested in hearing what others use.