Though the Neo Geo Pocket Color faded away before it got to build a full library, The Pocket Fighting Series made this system the best yet for fighting games.
The Pocket Fighting Series has a unique aesthetic of a applying super deformed chibi to SNK series characters. They all look about the same, but each one features different and exclusive characters and unique mini-games.
Though Match of the Millennium doesn't have "The Pocket Fighting Series" as its subtitle, I've included it here due to its blatant similarities.
Also, it should be noted that the European games are playable on US systems because they're region-free.