Ive been watching the Fable 2 stuff recently and of all the things that are getting me excited the gameplay trailers are REALLY doing the trick, so id have to vote for Gameplay.
Screenshots are getting old... no offense but weve been getting nothing but screenshots for a long time now... i think everyone wants some change...
Concept art just makes ppl raise more *** noobish questions, and also for the same reasons as above.
Cinematic trailers would be my 2nd choice cuz theyre usually really pretty, short and sweet. but they also dont always contribute to the game info, like the Starcraft 2 cinematic trailer didnt really explain anything... it just showed a marine suiting up...
Music... well music on its own kinda sucks... its just a single tiny element... it should be incorporated with something like a video.
so yeah im thinkning we need more actual game info, maybe not just story info but more gameplay info. like how is the game gonna be played?
so Gameplay trailers are possibly the best way to go. and besides they get alot more ppl psyched about a game so releasing one may inccrease your fanbase... and i dont mean the Halo fanbase but the Halowars fanbase there is indeed a difference.

I guess I'm a lucky lil Spartan...