C64 was, and will probably always remain my favourite computer system. It is indeed why I wanted a Pandora, a portable C64 is a childhood dream, now it's come true. Here's some games I really think are worth checking out, in no particular order:
Buggy Boy
Batman
Turbocharge
Dizzy (Yolkfolk is a good one to start with, Magicland is the best)
Spindizzy
Tilt (codemasters, once you get used to playing it's addictive)
Project Firestart
Bubble Bobble
Rainbow Islands
Creatures
Creatures II
Mayhem in Monsterland (over rated, but still great)
Sensitive (A PD game, some good clones on the Plus4 machine too)
Leaderboard Golf
IK+
Paradroid
Pitfall 2
The Sentinel
Newcomer Enhanced (change drive settings to get it working)
Live and Let Die (buggyboy in a boat)
Pang
Lode Runner
Space Rogue
Midnight Resistance
Krackout
Racing Destruction Set
X-Out
Mercenary
OK, there should be at least 3 games out of that lot to keep you occupied for a while.