I have very deffinite views on this!
Don't go DVD, unreliable, difficult to copy off and edit sometimes, can't take extreme conditions.
Hard disk are better, but not suitable for rough use, and you really need to take a laptop with you, or some other means of storing the films, otherwise lose the camera and you have lost everything.
SD card is good if you can afford, or are willing to carry around a good one. The cheap ones are fine for "snapshot" movies, but lacking optical zoom, are pretty limited in what they can do.
Mini DV is still the best all round. Tapes are cheap, they can take heat and knocks well, you can use a different tape each day (more than one if you like) and so not lose everything in one go. Easy to edit on PC with cheap or free software, and the cameras give the best performance for price ratio by far.
If you want something that works very well, and costs just over £100 (I paid 103 a month ago) have a look at the PanasonicNV-GS60. Very easy to use, and small and light. For about £10 you can get a big battery off ebay that will give 2 hrs recording.
I bought one because I wanted something small and cheap I could let kids use at school to save my bigger one, and I have been very impresed by it.
It has a 30X OPTICAL zoom, which is very useful out and about.
