Which Felicia should i Use?

I'd say +spd as she needs to activate Glacies to kill in many cases, and the spd will help her activate it faster

For low/no merges: +spd

For high merges or when she has reliable spd support thrn +atk is usually better

I'd go +Spd since everything is getting faster and she may end up getting left behind. I use her +Spd -Def and being able to double is key for her Glacies.

I have a neutral +2 Felicia and I'm saying +atk is an easy pick for me. I use her with a small amount of buffing and I often find that she's 4-6 damage short of securing a KO.

That being said, it does depend significantly on who she's playing with and what her role will be. If you want her to win against physical ranged units (which she actually can, but it's harder) using distant defense, then neutral is probably better.

I recommend a look through this guide:
https://docs.google.com/document/d/1pUHipAmv09FCn71Shh_Yks89c4YkCoMbmUuegF1A8C4/