I guess it is safe to say that I love the KOF games in general as 2D fighters... and a handful of other 2D fighters.
Specifically speaking I love KOF96, 97, 98, 2000, 03, XI, 02UM, and XIII...
Other 2D fighters that I enjoy are SFA2, CVS2 (despite the dumbass SNK character nerfs, even though I default to SNK character anyhow), and Skullgirls