One of my favourites for the Plus is Star Wars - a great vector graphics game that runs very smoothly on the Plus. Objective is, as you would probably have guessed, blast Tie Fighters and destroy the Death Star. Great sound effects too!
What kind of games do you like to play? If you're a strategy fan you've got to try Defender of the Crown, RISK and Strategic Conquest. Grab a buddy for RISK if you can. If you like Tetris type puzzle games nothing beats Pipe Dream (man, I love that game). If you like adventure/explore games there are TONS. Indiana Jones, Police Quest, etc. For arcade games there's not a better game than Dark Castle. There may not be a better game period. If you like Dark Castle try Beyond Dark Castle. Then if you find yourself a real Dark Castle fan get Return to Dark Castle for OSX. There really are a ton of games for the Plus - some that will only run on the Plus (like DC).