Welcome to RadioSEGA, if you are not a member register today!
If I had to choose a favourite, I'd say most of the Sonic the Hedgehog 2 soundtrack. Particular favourites would be "Green Hills Zone" which went on to become "Toot Toot Sonic Warrior" on Sonic the Hedgehog CD... I'm also fond of the Good Ending music which does not seem to be present on the Master System version.