Easily Killer Instinct (2013). The breaker system of matching your opponent's attack strength, counter breakers, and the lockout system forces you to pay attention to your opponent's tendencies and mix it up when doing your own combos. Then you've got manuals which are tough to break and require a guess, the difference in strength of linkers, etc. It's mentally exhausting but really deep and rewarding to have the combo game be an extension of the neutral game. Is he going to drop and reset? Is he going to fall into a pattern? Is he fishing for a counter breaker or a lockout? And when you hit it, C-C-C-C-C-C-COMBO BREAKERRRRR from the announcer that is probably more hype to be announcing the game than you are to be playing it. Also no resource cost, so if you happen to make a mistake in neutral you aren't necessarily going to get whomped with a 50% combo if you know your matchups or make good reads. It's a unique system that has really grown on me.