Now that I've actually had some time to boot up 64HDD, here's a few more selections:
Pinball Spectacular: One of the early titles of the C64, it was released in 1983. Think "Gee Bee" on the Commodore and you're almost there -- in fact, the game was developed by the now famous HAL Laboratories. Despite it's simplistic nature, this is a game I can play for hours, simply just trying to rack up points and break my previous high score.
Nebulus (UK)/Tower Toppler (US): A fantastic puzzle/action game that involves controlling your toady character up the side of a tower, climbing to get to the top. Along the way, you'll deal with moving platforms, floating eyeballs and other baddies. The 2.5D effect the game uses is great, and the gameplay is interesting as well. If you fall off, you don't necessarily die: You can still land on a lower platform, brush yourself off and continue on from that point.
Exolon: A nice platformer that resembles Jetpac in a passing glance, but the gameplay is drastically different. This game often has polarizing opinions: Some people dislike the game's difficulty, while others praise the unique graphic style and gameplay. I am firmly in the latter camp.
Jumpman: 'Nuff said.
"I am a cipher, wrapped in an enigma, smothered in secret sauce."
Talking abou Gee Bee
Gee Bee Air Rally (Activision), now this is a real C64 hidden gem.
I remember Gee Bee Air Rally getting an A+ rating in a magazine back when it was released, can't remember which mag it was. I also remember never ever finding a copy of the actual disk.
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A couple more:
Mr. Robot and his Robot Factory
Aztec by Datamost (Apple II port)
Tas times in tone town
Alter ego male and female
murder on the mississippi
borrowed time
LCP
night shift
Flimbo's Quest.
A good game, and probably some of the best music you'll ever hear come out of a C-64. The games opening tune is 20 times more catchy than the Mario Bros. theme.