Depends what you want, but MW2 OG had it all. Good maps, good vibes, no BS. BO2 is the fan favorite for a reason. Peak era of COD, no cap.
MW2019 brought the series back to life imo. Great campaign and MP.
World at War doesn’t get enough love. Gritty and brutal, especially in co-op.
Cold War was fun if you liked zombies and fast-paced MP. Not the best, but solid.