Loki comes at No. 3. This MCU is such a Villain who troubled Avengers many times.....
.....but after Loki Web Series, when this guy became Hero from Villain, it was not known.
No 2 :- Thanos, now this was such a Villain who had almost succeeded in his plan.
It destroyed half the world in a pinch and many SuperHeroes had to die to defeat Thanos.
Zemo comes at number 3, he is the only villain who was completely successful in his plan.
Its purpose was to separate the Avengers and it was also successful in this.
That is why he is the most successful villain of MCU till date.