The likes of Zool and Superfrog are great, but there is something extra special about Sonic The Hedgehog IMO. My eldest daughter played it when she was little and I thought it would be nice if my other daughter (6) could play it on her Amiga if possible.
Personally, I really enjoyed being the yellow Super Sonic and flying around on the Wing Fortress level of Sonic 2 :-D
I've just dug out the cheat on the page of a mag that I had stored inside the Sonic 2 cartridge in case anyone is interested.
Level select:
Go to the options screen and select Sonic on his own. Now go to the sound test and play the tunes 19, 65, 09 and 17. If you've done it correctly you'll hear a ping on the last tune. Return to title screen and select a one player game while holding down A. You'll now go to a menu with all the levels.
Super Sonic:
Get to the level select as above and go to the sound test. Play 04, 01, 02 and 06. Now when you collect 50 or more rings on a level, press B and C together and you will become Super Sonic.
:-)