I am afraid to answer, because this is obviously impossible to answer based on facts...
Personally I would recommend to pick movies according to your most liked genres first as they might keep you interested the most. It's probably near to impossible to suggest movies from one's own likings. But since you've asked - go for "Das Experiment", "Das Boot", "Napola", "Der Untergang". All do not contain too much action or special effects so that the dialogues are more easy to focus on. If you are into comedies I'd recommend the movies by Loriot, although I guess that the understanding of humore in a foreign language is a higher art. (Your call!)
Have a look at http://de.wikipedia.org/wiki/Liste_bedeutender_deutscher_Filme (a condensed english page is available too) for a list of - lets say - more distinguished german movies. This list of course is neither taking into account the complexity of language nor the mere existance of subtitles. So better check first.