This one should not be overthought:

1. Anything which is on the excluded list under the COC or is excluded under your travel insurance. While the exclusion list may not be applicable to international flights under the Montreal Convention, it's still a guide in those cases.

2. Anything which you need on arrival and cannot be easily replaced.

Also worth bearing in mind that most carriers and insurance only reimburse at fair market value. The shirt you bought for $150 and wore three times is not worth about $20.