I look at tiers like shelves, not a number order placement of characters because it's literally impossible to make a 100% definite numerical assessment of a lot of fighting games because there are a lot of top players out there who can play certain characters against others and not and vice versa.
These are unquestionable top tiers for the 2D MK series, they might even be a little generous, who is the very best is debatable, but there is almost always a definite #1 character in any game.
MK1: Sonya, Johnny Cage
MK2: Mileena, Jax, Liu Kang, Kitana
MK3: Sub-zero, Kabal, Nightwolf, Jax, Robot Smoke, Kano
UMK3: Kabal, Human Smoke, Kung Lao, Nightwolf, Robot Smoke, Kano, Jax, Ermac
MKT: Noob Saibot, Kabal, MK2 Kung Lao, Human Smoke, Nightwolf, Baraka, Robot Smoke, Rain, Kano, Jax, Ermac
There might be a few others here and there, but that's about it. The middle tier for UMK3 and MKT is pretty big, and if played right, a lot of the middle tier characters can give the top tier characters a run for their money.
Matt