Most everyone you will probably ask (other than hardcore game fans) will probably say that Pitfall! is their favorite Activision game because it is the most well-known and successful of their games.

Personally, it doesn't even rank in my Top 5 of favorite Activision games even though its a really memorable game and very fun.

What's everyone's favorite Activision game for the Atari VCS?

Mine is, of course, Pressure Cooker (which is very underrated)
I LOVE THIS GAME!!!!!!
Keystone Kapers comes in at a close second.