Pitfall 2, unbelievable game...
I remember as a kid walking into some record/video store in a mall (whatever those would have been in the early 80's - Goody's? - man, who knows) and there was a *wall* of these carts set up for xmas buyers (it was November). Hard to beleive they are considered somewhat rare today.
Custom chip for the music - so much better than anything else on the 2600 its not hard to understand why lots of folks still remember it (along with the music for Super Mario Bros. and I imagine a lot of other games...). That custom chip caused lots of problems for emulator programmers in the mid to late 90's - they got it pretty good now...
Pitfall 2, Pitfall, River Raid, Tunnel Runner, just about anything on the Starpath Supercharger (Dragon Stomper esp.) - yeah...nice...