Series as in share the title in common, I think that's the general rule, nothing else. Breaking down so much takes away from the real goal of this thread, what are the most favorite games out of the iconic series on the Neo.
KOF
FF
SS
LB
AOF
WH
Everything else is not solidified enough to be a series, not even Savage Reign and Kizuna Encounter. Not even the Double Dragons. Why would NM be along LB, makes no sense whatsoever, regardless of the team. Matrimelee cannot go along with anything else, not even ROTD, just 'cause is Noise Factory doesn't make it a series.
At the end making variations and intricacies steal the chance to observe the very favorites out of a solid line up of series.