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.