I'd have to say Contact and Das Boot were my two favourites, and I'd seen neither film before buying the DVDs.
Contact is a thought provoking story, superb video transfer and makes excellent use of 5.1. The first 2 minutes of the movie are worth the purchase price alone - especially if you want to demo your home theatre to friends! - and for the movie buffs there are 3 commentaries provided which should provide hours of fun. The SFX are superb and a lot would go unnoticed until you listen to the effects commentary.

As for Das Boot, this film absolutely drips with atmosphere, it's a great story, and the claustrophobic backdrop and sound is great - highlights include the shearing bolts of the sub under pressure! Again, there is a directors commentary and a making of extra.

Enjoy!