So the gift categories are as follows, along with the love points they give on challenge mode (assume that each value is doubled for story mode)

• Preferred (+15)
  
• Liked (+9)
  
• Okay (+4)
  
• meh (+1)
  
• Hate (none)

All girls usually have 2 preferred items!
Lotus Brooches, Jewelboxes and Friendship Cupcakes are universal +9 gifts
You can assume that everyone likes all of Serena's luxury shop items but each girl has 2 luxury items that they're only okay with

Firebird Feathers, Philosopher Stones, and Lady of the Lake Fish all universally give +100 points. RIght now, getting them seems to be a completely random chance. However, their drop spots are always the same.

• Philosopher's Stone : Castle Dog (it's an fma reference orz)
  
• Firebird Feather : Infirmary Drawer (makes sense since it's a phoenix down)
  
• Lady of the Lake Fish : Outskirts fishing rod

The Ocarina of Love universally gives a whopping 500 points. One thing to note is that it doesn't immediately end your playthrough so you don't have to worry about missing any special scenes when you give it... besides maybe chat options.

Remember that giving the same item to a girl twice in a row has diminishing returns on love points (usually half of the usual) so you need as much variety as you can, especially on challenge mode!

Your stats also play a part in gift giving!
Each girl has a stat that they like and you get bonuses equal to half that stat for Okay gifts
On the other hand, you get a bonus equal to that stat when giving out Preferred gifts!

Quick runthrough of each girl's preferred stat at least for gifts:

• Beatrice: Charm
  
• Vel: Strength
  
• Gisselle: Intelligence
  
• Amy: Intelligence
  
• Kamala: Strength

Birthdays don't seem to have any bonus effects on giftgiving. However, you do get special dialogue if you give a girl a preferred item on her birthday.

If you find any mistakes, please feel free to correct me~