The demo did Mercenaries 1 was pretty great. I think it even had cheat codes.
Glad somebody else remembers this. You got a good sized enemy base to screw around in and it did a fantastic job of showing off the game mechanics and destructibility. I played it repeatedly and the game went from being one I'd never heard of to a must buy.
There were some great ones for the Sega Saturn back in the day, including the entire first disc of Panzer Dragoon Saga and a Virtual On demo that let you use all the characters.
Yeah, it's hard to top the Panzer Saga demo. I remember in the letters section of the following issue, somebody actually wrote in to ask when they would be including the following discs.
Because it was the actual first disc of a four disc game, when you finally got the game, you could just carry on from where you left off. Thing is, I played the first disc so much just flying around the last area without triggering the Atolm fight I was hilariously over levelled and it made the rest of the game trivial.
I also agree on the Virtual On demo, it's the sort of game that sells itself when you see it in motion.
Other demos that stick out for me were two I played on friends PC, Diablo and Heroes of Might and Magic 3. So much fun to be had with both of those especially for two kids with no money for actual brand new, full games.