In general I think those beliefs need rebalancing, but that's just me.
1. Morrigan, Harbinger of Strife Earn Gold, Culture, and Golden Age Points from kills.
Given that you will play Celts as fighting nation (which certainly isn't the worst idea), this is by far the best belief. At the start of the game you will spawn Pictish Warriors like zerg queen and, since you should take authority, all of your little zerglings will produce faith, 2xgold, 2xculture, science and GAP on kill. You're able to get full policy tree and enhanced religion before anyone will even get a prophet.
2. Epona, the Great Mare Receive +15 Science, +15 Culture, and +15 Food when your Borders expand, scaling with Era.
This is very strong pantheon that allows your cities instantly to grow to 10-15 population turn by turn and works wonders with Tradition tree, as well as authority. But it's quite important to remember to beeline for Angkor Wat.
3. Bran, the Sleeping Guardian -+100% increase to Ranged Combat Strength, +25% Growth, and +1 Culture in every City.
Ok, so there is serious mistake in description here. RCS is only for cities, so don't think your crossbowmen will suddenly get machine guns and your trebuchets will become cannons. 25% Growth is great - that's like 33% of chinese UA! Culture is an icing on that quite decent cake.
4. Lugh, the Skilled One +4 Culture, +4 Science, and +4 Gold in Cities with a Specialist, +1 Culture from World Wonders.
Since sooner, rather than later, you'll get specialist in every city (workshop, market), that's essentially +4 of two of 3 most important yields in city. Quite good. +1 culture is nice, but not that important.
5. Dagda, the All-Father +1 Culture, +1 Gold, +1 Production and +1 Science for every 5 Followers of your Pantheon in owned cities.
This is a little bit better God-King. With 10 pop city you're killing Rhiannon or Mannanan.
6. Ogma, the Learned +1 Science for every 4 Citizens in a city, and +8 Culture when a Citizen is born.
Compare 16 pop city to Dagda. Ogma gives you +4 sci and gave like 120 cul. Dagda gives +3 sci, prod, gld and most likely already produced more culture.
7. Nuada, the Silver-Handed King +1 Culture for every 15 Gold per turn, and +1 Golden Age Points for every 10 Science per turn.
Assuming you'll get 200 gold per turn profit in medieval and 150 science, that's barely 15 cul and 15 GAP per turn. Totally not worth it.
8. Rhiannon, Goddess of Sovereignty +2 Golden Age Points, +2 Science, and +2 Gold in every City.
Looks good on paper and seems to work well with progress, but GAP are least useless of all and +2gold is valuable only early game.
9. Mannanan, Son of the Sea +2 Food, +3 Production, and +4 Gold in coastal Cities.
Ok, this is situational and I can imagine it working well with progress Celts, getting insane stats of newly found cities. But Celts don't have coastal bias, so we can assume it will work only for like half of your cities, which make it not as appealing.
10. Cernunnos, the Horned Stag +1 Food from Forests, +1 Production from Jungles.
This is so bad, I don't know how it passed any balance tests. At best (AT BEST!) you will work like 5 forest and 5 jungles in a city (but with time most of them will be chopped to get access to resources). That's 5 food and 5 prod in city of 15 population or so. 5f already in medieval is quite irrelevant. So only to get production from jungles? Yeah, early on, this may have some appeal, but it scales so badly, it's not worth EVER.
1. Morrigan, Harbinger of Strife Earn Gold, Culture, and Golden Age Points from kills.
Given that you will play Celts as fighting nation (which certainly isn't the worst idea), this is by far the best belief. At the start of the game you will spawn Pictish Warriors like zerg queen and, since you should take authority, all of your little zerglings will produce faith, 2xgold, 2xculture, science and GAP on kill. You're able to get full policy tree and enhanced religion before anyone will even get a prophet.
2. Epona, the Great Mare Receive +15 Science, +15 Culture, and +15 Food when your Borders expand, scaling with Era.
This is very strong pantheon that allows your cities instantly to grow to 10-15 population turn by turn and works wonders with Tradition tree, as well as authority. But it's quite important to remember to beeline for Angkor Wat.
3. Bran, the Sleeping Guardian -+100% increase to Ranged Combat Strength, +25% Growth, and +1 Culture in every City.
Ok, so there is serious mistake in description here. RCS is only for cities, so don't think your crossbowmen will suddenly get machine guns and your trebuchets will become cannons. 25% Growth is great - that's like 33% of chinese UA! Culture is an icing on that quite decent cake.
4. Lugh, the Skilled One +4 Culture, +4 Science, and +4 Gold in Cities with a Specialist, +1 Culture from World Wonders.
Since sooner, rather than later, you'll get specialist in every city (workshop, market), that's essentially +4 of two of 3 most important yields in city. Quite good. +1 culture is nice, but not that important.
5. Dagda, the All-Father +1 Culture, +1 Gold, +1 Production and +1 Science for every 5 Followers of your Pantheon in owned cities.
This is a little bit better God-King. With 10 pop city you're killing Rhiannon or Mannanan.
6. Ogma, the Learned +1 Science for every 4 Citizens in a city, and +8 Culture when a Citizen is born.
Compare 16 pop city to Dagda. Ogma gives you +4 sci and gave like 120 cul. Dagda gives +3 sci, prod, gld and most likely already produced more culture.
7. Nuada, the Silver-Handed King +1 Culture for every 15 Gold per turn, and +1 Golden Age Points for every 10 Science per turn.
Assuming you'll get 200 gold per turn profit in medieval and 150 science, that's barely 15 cul and 15 GAP per turn. Totally not worth it.
8. Rhiannon, Goddess of Sovereignty +2 Golden Age Points, +2 Science, and +2 Gold in every City.
Looks good on paper and seems to work well with progress, but GAP are least useless of all and +2gold is valuable only early game.
9. Mannanan, Son of the Sea +2 Food, +3 Production, and +4 Gold in coastal Cities.
Ok, this is situational and I can imagine it working well with progress Celts, getting insane stats of newly found cities. But Celts don't have coastal bias, so we can assume it will work only for like half of your cities, which make it not as appealing.
10. Cernunnos, the Horned Stag +1 Food from Forests, +1 Production from Jungles.
This is so bad, I don't know how it passed any balance tests. At best (AT BEST!) you will work like 5 forest and 5 jungles in a city (but with time most of them will be chopped to get access to resources). That's 5 food and 5 prod in city of 15 population or so. 5f already in medieval is quite irrelevant. So only to get production from jungles? Yeah, early on, this may have some appeal, but it scales so badly, it's not worth EVER.