If you mean a demo from the devs in a Video,
then id say online gameplay, but........... a little more fleshed out then what we have seen in the vids so far, like show a proper team working together or hell even invite 2 well known clans from say W:ET or quake wars to come play the game, then get them to play a proper comp match that is videoed / edited with commentry from devs explaining things etc etc , not just some pub randoms from quakecon / E3 or any other show room floor playing. (this will probably never happen as who would pay flight costs etc etc

.. we can always hope tho )
Also I'd like to actually see the PC version being played instead of the 360 one. The online gameplay vids ive seen so far are all from the 360 or PS3, so the feel is all weird to me in those vids with all the gamepad aiming going on, it just makes me cringe.
If you mean an actual demo of the game that people download and play themselves.... then id also have to say online gameplay.

Mainly because of the fact that when I tried out the Quake wars demo, which was only 1 map online/offline, I got hooked.
I swear I played hour after hour after hour of that Valley map with my brother online, constantly watching the forums / website for more news until the game came out and I bought the SOB right away.
It was a pity it didn't get the audience it deserved ( some say the demo came out too early and burnt a lot of players out which is probably true to a point......not me tho

) as it was a great game, not as good as W:ET but still had that.....W:ET flair to it.
I would play the brink demo for hours on end just the same if it were online