Did a little research, figured I'd share, though this has probably been posted before

There are 4 reactions a companion can have to a gift, from best to worst they are: Favorite, Love, Like, Indifferent

Companions also have levels of affection:

Lvl 1: 0-2000

Lvl2: 2000-4000

Lvl3: 4000-6000

Lvl4: 6000-8000

Lvl5: 8000-10000 (max)

You can find guides online, but I recommend simply buying one each of the gifts from the companion vendor and giving them to your companion, because all of the guides are wrong in places.

And thankfully, there is a formula.

If you give them a gift of a level appropriate to their faction (i.e. if your companion has 3000 affection, you want to give them lvl 2 gifts), you get:

96 Affection for Favorite Gifts

54 Affection for Gifts they Love

24 Affection for Gifts they Like

Nothing for gifts they’re Indifferent to

Q: What happens if you give them gifts that are too low level?

A: Divide the reward in half for every level it’s too low.

Observation: Generally you don’t want to give them too low lvl gifts, maybe 1 level lower but probably not more than that.

Q: What happens if you give them gifts that are too high level?

A: Add +10 affection per level it’s too high.

Observation: Save gifts that are too high lvl for later.

Oh, and Blue gifts give 2x Affection, and Purple gifts give 4x affection.

Lvl 1 gifts you can get from the vendor for 200 or from quests for 100 (though you can get exactly what you want from the vendor), or blue/purple for 300 through quests

Lvl 2 gifts are 600 from the vendor, 200 from quests, 600 for blue/purple through quests

Lvl 3 gifts are 400 from quests, 800 for blue/purple through quests

Lvl 4 gifts are 600 from quests, 1200 for blue/purple through quests

Lvl 5 gifts are 1000 from quests, 2000 for blue/purple through quests

TLDR: Always give your companions their Favorite Gifts of Lvl = 1 + (1 for every 2000 full affection you have with them)