It's quite fun to play your old favorites but I think fanbased ROM hacking is where emulation really shines.

And here the enduring classic Sonic the Hedgehog is well represented with it's strong fanbased community and the amazing variety of [ROM hacks]http://info.sonicretro.org/Category:Hacks available. Everything from character replacement and clever level redesign to alternate plots and humorous premises breathe new life into this platform classic and make it easy to find something new to enjoy all over again.

Sega seems to tolerate if not embrace the Sonic hacking community, perhaps realizing it only adds to the cult classic status of their popular franchise and Sonic ROM hacks are distributed complete without patching. After playing some of them you might even decide to try your hand at Sonic hacking with the many tools and tutorials out there. The Sonic hacking community is quite vast with surely the most ambitious Sonic fanbased project of all being the much anticipated Sonic 2 HD which undertakes the creation of a completely faithful (non-emulation) remake of Sonic 2 using high definition graphics and a custom game engine.

Arguably the best Sega emulator is Fusion and download the Fusion Plugins too as this will add more video render options.

TLDR; Rom hacks are the best. Whatever your emulation favorite, find out if there is a loyal fanbase out there customizing it right now and if there isn't start one!