To put it simply, Killer Instinct is easily the best game in the series and almost everything about it is awesome.
Mortal Kombat is a gruesome and violent reboot of the series and underneath all the blood and guts is a fantastic fighting game.
Shadow Warrior is a solid reboot with a humorous protagonist, brutal combat, and an addictive upgrade system.
Painkiller: Hell & Damnation is a fun ride if you obtain all of the DLC. The base game alone is still fun but also a very condensed version of the original Painkiller.