Chaldea lunchtime

It works on all servants, no matter who equipped it and is better to limit break it only when you have all of them, because limit breaks won't inrease bonus until you get 5 copies

It's 2% bond EXP for each copy, which is 10% for all 5.
LB'd Lunchtime gives 10% bond EXP off the bat and it only takes up one slot. It's best to equip it on your back line servant if you intend to farm bond points because your front liners would likely have to use functional CEs in order to make your runs shorter and be more efficient. It also stacks with your support's Lunchime (Wavers would usually carry this CE) and your own +50 bond points CE, like the Portrait CE we got a year or so ago. That's the most extra bond points you can get out of combination of CEs.