This answer doesn't win the contest at all, but I include it for interest's sake and the hope to inspire future readers to play with the hardware.
The Commodore VIC-20 shipped with 5 kB of RAM and officially expanded to 32 kB, but very easily, and even using hardware available in the first year or so of its existence, it could be expanded to 40 kB (3 kB expander or SuperExpander [the latter of which had BASIC language enhancements], 16 kB expander, and 2 x 8 kB expanders [with their DIP switches configured appropriately so that the RAM doesn't overlap). That's an 8x multiplier over the stock RAM. Unlike some of the other competitors, all VIC-20s shipped with 5 kB so this would be a very normal multiplier.
Today, it's even easier to expand a VIC-20 as there are many, many RAM expanders available. 32 kB expanders are very easy to find, and adding a 3 kB expander and a cartridge slot expander will attain the same result. (There may even be single-cartridge ways of adding the entire 35 kB additional RAM required, but if there are, I'm not aware of them.)