MK2 is the easiest of the series, you can cheese the hell out of the AI with Scorpion, Reptile, Sub-Zero, Jax, Tsung, Baraka, and Cage. It involves you waiting low for them, jumping back when they get close, they follow, you punish. Either Scorpion's spear, and uppercut, or a roundhouse kick. Kung Lao, Raiden, Mileena, Liu Kang, and Kitana can't seem to cheese the CPU AI the same way.
UMK 3's/ MK Trilogy's AI can be cheesed another way, but not as simple a way. I've seen folks jump back, CPU throws a projectile, they jumpkick the AI or combo them as the projectile will miss and leave the CPU wide open (the jump in combo that started with UMK3 is handy). I've never been able to cheese the UMK3 AI, but I can do so with MK1 and MK2 with no issue, been able to 1CC both games enough times, or do it in 2 credits.
Noob Saibot and Rain are hella broken in MK Trilogy, easiest characters to beat the game with on the PS1. N64 MKT, both aren't as cheesy to use.