MK 1-3 are classics. UMK3 is the gold standard for competitive play in the classic series and still has a scene to this day, and is just crazy fun.
MKII meanwhile is just peak classic MK vibes in the art, music, and presentation. I have a massive soft spot for it having grown up with it, and it's the first one I think when I picture MK
Shaolin Monks is awesome if you want something more Beat-Em-Up Action Adventurey. Super fun if you like classic God of War styled games, and great for Co-op play too.
MK9 and X are awesome for the more modern games. MK9 is just one massive love letter to fans, with every character from MK 1-3 returning and super fun and rewarding (if a bit busted sometimes) gameplay.
X meanwhile is probably the deepest game in the series from a gameplay perspective. Super fast, freeform, and deep, it's probably the most celebrated MK by the FGC and still beloved.
There are other good games in the series too, but these would be my top recommendations as a fan.