As far as bosses, I believe Kahn owns all.

As far as broken, I usually use the word referring to a character that has abilities or characteristics that don't fit with the limitations imposed on the rest of the characters. For instance, Noob Saibot is very broken. All of his movements are faster and his disabler projectile is super fast with almost no startup frames. That breaks him.

Similarly, MK2 Kung Lao is broken because of the damage he incurs and his speed. His combos are very easy, do a lot of damage if they connect, do a lot of damage if they are blocked, and can lead to infinites, or just long juggles. Also, the speed is crazy. You can actually combo his autocombo off his sweep. It is blockable but probably would take someone off guard because it connects at all. He is intentionally broken because all of these things were put in intentionally. He does MK2 level damage. That is ridiculous. Baraka, Johnny Cage and MK2 Jax also do MK2 level damage, but none of them have the speed, specials or combo abilities that MK2 Kung Lao have so they end up being fairly well balanced as far as this is concerned.

On a side note, MK2 Raiden does not have MK2 level damage. Don't know if he was transfered to MK3 levels of damage earlier on as a potential character to put in Ultimate or something nifty like that, but the end result is he is far more balanced then MK2 Kung Lao (he is a little slower as well).

Broken can also mean easily exploitable moves, like Rain's simple infinites in MKT. In UMK3 I believe Kung Lao was pretty broken too because of his spin infinite in the corner.

However Infinites do not always mean broken. For instance Sindel's flight cancel infinite is so hard to pull off that it is far from character breaking.

One of the things that makes the bosses so incredibly deadly is that they take no chip damage from combos after the first hit, and with a starter, after the second hit which was previously the first hit. Another point to mention that is related to this, bosses can block mid combo after any hit, so if you start Sheeva's 7 hit combo Shao Kahn while he's recovering from an attack, as long as they are holding block, he will block the last 6 hits of the combo. Another blocking related issue with the bosses is they can block and do regular attacks while holding block so there is absolutely no time inbetween the attack start up and coming out of block. With normal boss damage on, it helps some characters because they then take chip damage. You would think putting on normal boss damage would change the MK2 characters as well, but it doesn't, they still do MK2 damage.

When all is said and done, the tiers are pretty much only accepted in terms of normal characters without the bosses even being considered for tourny play. Noob Saibot is generally banned as well.

Dreemer covered everything else very well.

Matt