Sadism - Dr. Mundo sacrifices a portion of his health for increased movement speed and drastically increased health regeneration.
Dr. Mundo regenerates 50% of his Max Health over 12 seconds. Additionally, he gains 15/25/35% movement speed.

Cost
Costs 20% of Current Health
Range
20

Highlander - Master Yi moves with unparalleled agility, temporarily increasing Master Yi's movement and attack speeds as well as making him immune to all slowing effects. Additionally, killing a Champion refreshes all of Master Yi's cooldowns.
Increases Master Yi's movement speed by 40% and Attack Speed by 40/60/80%, and he becomes immune to all slowing effects for 6/9/12 seconds. Additionally, killing a champion refreshes all of Master Yi's cooldowns. (Assists reduce half of the base cooldown amount.)

Cost
120/120/120 Mana
Range
1

why those ultis are so unbalanced? i mean dr mundos ulti only get 10% movement speed when it lvl up, while the yis ultimate get duration an atack speed, plus u cant be slowed u get reduced cool down when u kill or do an assist, dr mundos would be a cool ultimate if those 50% healing couldnt be lowered by some common debuffs like ignate and if it had some other bonus when u level up it, only 10 % movement speed its not enough.

IMO

Mundo: Ignite, Kat's bouncing blade (with her passive on it), Tristana's active use of explosive shot, Gangplank's passive all solve his regen, and really, unless Mundo pop's ghost, he's not moving that fast, since he almost absolutely REQUIRES Mercury Treads.

As far as Yi is concerned, Exhaust, Teemo's blind, Corki's blind, or just plain CC eliminates his effectiveness. Failing that, buy a thornmail, and watch Yi explode himself FOR YOU.

Anything else I can help you with this morning?