You rarely see historical ratings for dayparts other than prime time. Here's the top 5 Saturday morning shows for each network for the Jan.-Mar. 1968 Nielsen Television Index as reported by Television Age Magazine:
ABC
1. Journey To The Center Of The Earth--8.5
2. The King Kong Show--8.4
3. Spider-Man--7.9
4. Fantastic Four--7.8
George of the Jungle--7.8
CBS
1. Jonny Quest--9.6
2. Superman/Aquaman Hour--9.6
3. Shazzan--9.2
4. Moby Dick and Mighty Mightor--8.9
5. Roadrunner--8.9
Space Ghost--8.9
NBC
1. Flintstones--7.3
2. Samson & Goliath--6.8
3. Top Cat--6.1
4. Birdman and The Galaxy Trio--5.9
5. The Super 6--5.3
Atom Ant/Secret Squirrel--5.3
For the 1967/68 season:
CBS had a 8.5/37
ABC had a 6.9/30
NBC had a 5.9/26
Article also mentioned that NBC had high hopes for next season's Banana Splits to bring it out of last place.
ABC
1. Journey To The Center Of The Earth--8.5
2. The King Kong Show--8.4
3. Spider-Man--7.9
4. Fantastic Four--7.8
George of the Jungle--7.8
CBS
1. Jonny Quest--9.6
2. Superman/Aquaman Hour--9.6
3. Shazzan--9.2
4. Moby Dick and Mighty Mightor--8.9
5. Roadrunner--8.9
Space Ghost--8.9
NBC
1. Flintstones--7.3
2. Samson & Goliath--6.8
3. Top Cat--6.1
4. Birdman and The Galaxy Trio--5.9
5. The Super 6--5.3
Atom Ant/Secret Squirrel--5.3
For the 1967/68 season:
CBS had a 8.5/37
ABC had a 6.9/30
NBC had a 5.9/26
Article also mentioned that NBC had high hopes for next season's Banana Splits to bring it out of last place.