One of my fond memories of "good old days" gaming is with the Battlesystem. I (as DM) set up two friends with the "make your own army" guidelines at the back of the book. I gave them each a 1,000,000 xp budget and said "any published book, no limits".
Oh man, were they devious.
One friend's army included invisible magic-users (each with Disintegrate scrolls), fifty invisible silver dragons and a horde of illusionary iron golems.
The other delved deeply into the Fiend Folio, bringing retrievers, drow sorcerers on nightmares and all manner of slaad.
Easily 80% of the participants were invisible to start, making for a very empty board.
The illusionary iron golems (appearing to be hundreds of them) eventually drew out all the opposition as they "advanced", making units vulnerable to the real attack of silver dragons, etc.
It was a nonsensical slug-fest, but great fun for all. Probably 15 years old, using mostly counters and other random minis for the armies. We still talk about it. :)
