it's been pointed out that the 7800 gives you two systems and you have an SMS if you have a Mega Drive, but it's worth pointing out that the SMS can also play SG-1000 games, which the MD cannot. the SG-1000 has a rather modest library but its very similar to both the ColecoVision and MSX1 to the point many of both console's heaviest hitters have been ported over by fans. getting all of that PLUS the SMS library makes it the obvious choice to me IF you don't already have an MD to play SMS games on. if you do, despite my bias i do have to say that both the NES and 2600 (via the 7800) both probably have deeper library then just the additional games SG-1000 support gets you

if you're torn, a ColevoVision with a RAM expansion and a 2600 adaptor gives you the SG-1000/coleco/some MSX1 AND 2600 support, and if you get a Super Action Controller too you will be the fashion envy of retro gamers everywhere. decisions decisions decisions...