Depends on your definition of "highly valuable" - from that time period, there's a very short list of cards worth >$1000, quite a few in the $100-$999 range, and a ton in the >$10 bracket. What they're actually worth depends a lot on the particular printing and what condition they're in.
If you wanted a starting point, Scryfall is a useful tool for looking up cards (though they're missing pricing data for some early cards, presumably due to scarcity of transaction data). Here's something to get you started (cards printed before 2000, sorted by price, displayed as a price list): https://scryfall.com/search?q=unique%3Aprints+sort%3Ausd+dat...
If you wanted a starting point, Scryfall is a useful tool for looking up cards (though they're missing pricing data for some early cards, presumably due to scarcity of transaction data). Here's something to get you started (cards printed before 2000, sorted by price, displayed as a price list): https://scryfall.com/search?q=unique%3Aprints+sort%3Ausd+dat...