My favorite Atari 2600 game is certainly this one:

Such a simple and yet challenging game.

I also like Pitfall and Tennis.

I played a bunch of Combat, Yars Revenge, Maze Craze, Fast Food, Stampede, Joust, Centipede, Basketball, and so much more. I still have around 80 games and 2 of the consoles my parents bought for me and my brother in the collection. It was what I played from 1980 to 1989 until we were able to afford an NES. I played all of my games quite a bit, but the ones I mentioned popped into my head as being some of my more well-liked.

Invisible tanks on Combat.

Was awesome. Miss it.

Still have an old wood style 2600 and the plastic metallic strip looking one too. Have a paddle controller still in the box also. Was my favourite console when I was super young. Hand me down from my dad.

As the first action-adventure video game and first console fantasy game, Adventure (1980) was my all time favourite, there are many great games but this one really stood out 

Featured as the last clue in 'Ready Player One' movie (The first Easter Egg ever in gaming was in Adventure)

Actually, there were Easter Egg's long before Adventure. It was just that the nomenclature was not fleshed out yet. 

Pitfall 2. Gigantic game world. Lots of collectibles. Just an amazing step up from games at the time.

Missile Command probably my favorite.

Others are Chopper Command, Pitfall obviously, the earlier Basketball game, Super Breakout, Combat. The Pac-Man port was pretty crude, even for 2600 standards, and sadly I never played Adventure, probably would've liked that one.

Pitfall and Pitfall 2. I remember finally beating the first level in the original Pitfall. It was the first time I had ever accomplished something noticeable in a video game. I was 2 years old, I think. That started a lifelong love for video games.