Everdrive MD or Mega Everdrive.
If you require FM synth with your SMS games, either the NeoMyth MD flash cartridge, or an FM PowerBase Mini converter, a Genesis and a SMS Everdrive or original cartridges. Or, if you don't mind having both a SMS and Genesis sharing the same shelf space, a Model 1 SMS and a FM module from viletim.
The FM Powerbase Mini is an awesome product. You haven't heard SMS games until you experience the FM sound versions.
I don't own any flash carts (not for moral reasons, it's just I already had most of my collection when those started to become affordable) so I keep a Genesis and an SMS hooked up. I'd go with a Power Base but then I wouldn't be able to use the 32X as well.