Tetris - unmatched classic, hours of addiction guaranteed
Super Mario Land - I love this little old bugger
Mario Tennis - the new one , not the GB oldie. Incredible addictive thingy for your GBC.
Super Mario Bros Deluxe - Just as the NES jobbie. Excellent conversion
Metal Gear Solid - Excelent game, just like the good ole MSX games.
Those come off the top of my head, will post mre later. Long time no GB playing.