I'm making a list based on the most popular and competitive CAG games for me to play (Being a competitive person, I want to focus mainly on the most popular and played games). In terms of CAG games, I would consider the following 6 games to be the "Titans" in terms of a combination popularity and competition, With Donkey Kong being the clear number one, and Galaga at number two. Those two games (along with Donkey Kong Jr.) have websites (such as this one) that involve competitive play (I'm aware of TG and MARP, but those sites are broad-based, not that there is anything wrong with that).
=====The Titans=====
Donkey Kong
Donkey Kong Junior
Galaga
Ms. Pac-Man
Pac-Man
Robotron: 2084
I made a list of 50 other games that may fit the criteria as games just below the "Titans".
1942
Asteroids
Asteroids Deluxe
Berzerk
Bomb Jack
Burger Time
Centipede
Crazy Kong
Crystal Castles
Defender
Dig Dug
Donkey Kong 3
Elevator Action
Frogger
Galaxian
Gorf
Gravitar
Gyruss
Joust
Jungle Hunt / Jungle King
Kangaroo
Mappy
Marble Madness
Mario Bros.
Millipede
Missile Command
Moon Patrol
Mr. Do!
Nibbler
Omega Race
Pengo
Phoenix
Pole Position
Pooyan
Popeye
Punch-Out!!
Q*Bert
Qix
Scramble
Space Invaders
Super Pac-Man
Tapper
Tempest
Time Pilot
Track & Field
Tron
Wizard of Wor
Xevious
Zaxxon
Zoo Keeper
Based on this list (and it is just my opinion), I think the next 10-15 that are just below the titans. would probably go something like (in alphabetical order):
Asteroids
Berzerk
Centipede
Crazy Kong
Dig Dug
Donkey Kong 3
Frogger
Galaxian
Joust
Mr. Do!
Pole Position
Popeye
Q*Bert
Space Invaders
Track & Field
If you have a list of your own or have any feedback, that would be great.