Find most valuable NFT owned?

How can I find out what the most valuable NFT is that I own?

Currently, there is no direct way to get this data for a particular NFT Id.
You can use the NFT API to get the lowest price or last traded prices of an NFT in a contract.

In another way, you could opensea API to see the price bids on your NFT and consider the highest bid as your NFT value.

How do I get last traded price for an NFT?

Using GetNFTTrades from NFT API.

Go through the documentation for more details.