My family recently unearthed our Sega Genesis and once again we can not get enough of Sonic The Hedgehog.  We have a pretty diverse collection of Sega games, but this one always seems to rise to the top.  As far as the 'fun factor' goes, I'd easily stack it up against a fair percentage of my PS2 library.  I don't know much about the ports, except to say that Sonic has been made available on the Playstation, the Xbox, and even made portable for the Game Boy Advance.  I haven't played any of these, but the reviews I have seen are fair, at best.  So, proceed with caution.