Depends on the situation(weather, group ride/solo/charity ride?, did I charge the battery last nite? how's my memory card?).

Samsung Galaxy S smartphone:
https://www.youtube.com/watch?v=qdA_...6zPoymgKaIoDLA

Fujifilm XP50 waterproof point and shoot/pocket camera:
https://www.youtube.com/watch?v=Cqxn...6zPoymgKaIoDLA

ContourGPS sports cam. Small size; can mount on bars, fork, seatpost, rear traingle, top tube, etc.:
https://www.youtube.com/watch?v=Jea4...IoDLA&index=22