Which of my servants has the best bond CE?

Okay. Normally Self-Buff bond CE is not recommended since their CE is normally not stronger than equip SSR CEs and even some SR CE. Only very few servants have amazing self-buff effect from their bond CE, like guts type bond CE for Herc, Spartacus.

Team-Buff Bond CE are more helpful for game-play. If 2-3 team member equipped the same effect bond CE, then Art > NP gain > Quick >= Attack >= Buster > Critical >= NP damage and Star Generation. But it is strongly depending on team composition.
Healing and Team debuff-resistance up are also quite unique, showing excellent tanky effect.

My suggestion is to obtain bond CEs as much as possible then consider how to make bond CEs effective in specific team. Right now only Herc, Geogies, Arash and Astolfo (maybe more) show "metagame" beneficial of single bond CE. But that is still not enough.
For instance Liz's CE is great, typically if you pair her with Marie and Hans (all with bond CEs) for an tanky-debuff team. But only single Liz's bond CE won't shown such effect.
Similarly, as I pointed out "Art +15% is better than others", but it worked you have at least 2 servants with Art +15% bond CE in team of 3, and in specific Art team, otherwise it is even not better than if you equip them with 3 Projection CEs, right?\
So no need to consider too much to polarize bond CE for specific servants except your beloved.
If you strongly need a recommendation, I will choose Liz.