bit of a bump, but this was still on the first page, so… yeah, kof forums!
either kof98 or kof98um. 98 has a better and older playerbase and is easier to access right now, simple as that. 98um has a lot of cool changes in theory (some big ones: they made extra good and made the usa team mid to top tier), though since not many play it (yet, if final edition on steam gets anywhere) it isn’t really clear if those changes don’t go too far.
the thing with xiii (and, almost sadly, xi) is they’re different kind of games; one that’s still very good (well, xi at least), but one that’s not really kof. the thing with kof2002 is that it’s just not as good of a game as 98 (…or most of the games in the series, really), though i don’t know much about 2002um. and i can’t just say “screw kof, get kofxi” because kof is a pretty good game too!
(obviously this is an answer to the question by itself and not necessarily an answer to the op)