Which Martial Art style is easiest to learn?
Any one or several forms of fighting and self defense techniques that are practiced as sports are termed as Martial Arts and there are a wide variety of martial art styles to learn from such as karate, judo, taekwondo, Krav Maga and many more.
Martial Art is not easy to learn as one has to have command over his body movement skill and patience as well as speed.
Which form of Martial Art is easiest to learn is a little juggling question because learning an art depends on a lot of aspects, one of them being the country where you reside. Your age, your physical attributes and your passion towards an art also play a contributory role to the decision.
even though it might not be the exact I go with Taekwondo or Krav Maga. But ultimately it is all a personal choice and your requirement that makes any art easy or difficult.