My 2600 favorites that still hold up well:
Kaboom!
The secret to what makes this game great is the paddle controller. The game gets faster and faster, and when you learn to keep up with it, it's awesome!
Crossbow
A port of the arcade game with good graphics. Hapless adventurers walk across the screen and you're the crossbow sniper that must move a cursor around to shoot the enemies that attack them.
Plaque Attack
Kind of like a fast-paced, dental version of Space Invaders. Various food items swoop in to attack your teeth, and you blast them with toothpaste.
Crackpots
Bugs attack your building, and your best defense is apparently dropping flower pots on them.
Warlords
Also based on an arcade game. With 1 player it gets a little dull, but with 4 players it comes alive as a competitive version of Breakout.