Technically, for those of you who said "Tasmanian Tiger" or "Tasmanian Wolf" while they PWN, they aren't prehistoric. Extinct, but not prehistoric. Prehistoric means before written records...
Anyway, I was always (for some reason :O) a fan of Ankylosaurus.

I dunno why. I had a lot of plastic dinosaur toys as a kid (despite being a girl XD) and I think that was my first/favourite.
And I also thought Spinosaurus was pretty rad.

T-Rex to me always appeared over-rated. The big head, tiny arms... don't appeal to me.
Diplodocus was cool too.
EDIT: How COULD I forget... Iguanodon! Ooohy loooong thumbnails...

Images are (c) to whoever drew them. :/