Personal Motivation
One of the most important factor when choosing a second language is up to you. The easiest language to learn is the one that you are most motivated to learn, the one you enjoy speaking, the one with the culture that inspires you and the history that touches you spiritually. It is useless to try to learn a language if you are not interested in the people who speak it, since learning a language involves participating in its behaviors and identifying with its people.
Consider all three factors: motivation, accessibility and linguistic ease, in that order, and come up with the final list yourself. The bad news is that a second language is easier to learn the younger you are. The good news is that we humans are hard wired for a great amount of linguistic flexibility, as long as we know how to turn on the learning process. If the rewards and benefits of the language are clear to you, you will be able to get those rusty language synapses sparking in your head and start the words rolling.