OG KOF98/KOF98UMFE. KOF02UM is a close second. KOFXIII is a somewhat distant third. For two reasons. The first reason is because I played KOF98 a lot through out most of high school years starting around 06 and frankly... it always puts me in a good mood, especially if I am able to play with others. It pleases me on both the aesthetic and game play, which leads to the second reason: The game is quick. Very quick paced. Between two top (or, at the very least, intermediate) players, there is always something going on in the match. While the game generally favors the offensive player, it still offers the player different ways of playing the game and they will be able to play the game how they wish in order to win. The general game mechanics, different modes (although EXTRA did not really become that useful until UM came around), the cool glitches that are possible to exploit for going for the win, mobility options (one of the most important aspects), the characters themselves, the relatively simple combo system and defensive options is what makes the game so good. This is also part of the reason why KOF02UM is an extremely close second (I do not mention OG 02 because... I've only played in two tournaments with that title in Germany and never really owned the game. As for KOF02UM, I didn't own that game either until recently via steam edition. I found it to be highly enjoyable on both the aesthetic and competitive level). KOF98 (including it's other iterations, 02/02UM and some other older titles) rewards the a lot of understanding the neutral along with well-positioned normal moves while taking advantage of the spacing and the mobility options available.