I have a few most hated champions, and since I cant decide who are the top 2, i will do all of them.
Darius+Draven=Drarius.... Bleed passive, he throws Decimate, his axes that knock you side now pull you back, and his on-hit cripple now gives him a speed boost.... plus true damage ult that deals true damage to every target, then comes and does more true damage.... GG.
Karthus+Tryndamere=Karthamere.... OMG! His Q now deals damage and heals him based on his current mana, he lowers your attack damage, attack speed, and move speed with wall, and he now spins towards you with defile, and can spin away to deactivate it.... but the worst is his ult.... Upon activation, he casts Requiem, then becomes unkillable for a short time, regains a large amount of mana, and then for his passive, he gains bonus AD at lower health levels, and when he dies, he stays as a spirit, can still spin with defile to you.... Now thats GG.