Which travel insurance is best to cover for engagement rings for overseas proposal?

If you want a specific clause for the engagement rings, then you need to speak to an insurance broker or to the insurer directly to have that item specifically mentioned in the policy.

That means the insurer would have a look at the item, the receipt and provide the valuation in the specification of the policy.

As you can guess, it is going to be a lot more expensive than off-the-shelf travel insurance policies. so unless your ring is a big 5 figure, you may not want to take this route.

The 2nd way is to buy off the shelf policies, go for the most premium type, and do keep the receipt carefully at home (for claims in the future).
When you are overseas, do keep track and do document where the ring is. At any point, if something bad happens, make sure you report it to the local police, and have a proper record if the ring is stolen or whatever. This will help claims in the future.

Do note that such items of value in standardised travel insurance would probably have an excess, so you won't be able to claim 100%.

Have a great marriage proposal and safe travels!