Most Helpful Girls
There is no most important THING. It's a bunch of things that are important in a relationship.
Trust, Communication and of course Love. Without those 3 things a relationship will just die, maybe not at first but it will slowly fall apart over time which will just end up sadness :(
Most Helpful Guys
It sounds shallow but the real answer is physical attraction. Physical attraction to each other is the foundation that relationships are built on.
Communication is super important but good communication means nothing if two people are not attracted to each other. Personality is also super important but again, nobody is going to care about your personality if they are repulsed by your appearance.
Things like communication and personality are extremely important for a relationship to be healthy. However the relationship will never begin in the first place without physical attraction.