Sega Master System / Mark III / Game Gear
Sega's rendition of the world famous board game has all the features of the original and sports some colourful graphics. The trouble is, that it's slow to play. It's a shame that this is the case, since it's pretty good fun, but it just gets too dull when you're waiting for the computer to do its stuff.
Unless you're desperate to play Monopoly and never have a friend to play with, you might as well buy the actual board game - it's cheaper, and is a lot more fun than this (after all, you can't argue with the computer!)