The easiest foreign language to learn is one that shares common roots with your own. Most languages fall into certain groups. It is relatively easy to study a language similar to your own, or similar to another language that you already have some experience with. For example, French and Spanish are close cousins of English. You will probably find it easy to deduce the meanings of many words on your own. If you have studied a little French or Spanish, then you should not have any trouble transitioning to Italian. If you study one, then switch to any of the others, you will find remarkable similarities.
Even if the words seem completely foreign, the sentence patterns will be the same. At some level, you will have an intuitive sense of how things go together. This makes it much easier to learn a closely related language rather than one from a unrelated lineage such as Swahili or Japanese.
Are their many resources available in your target language? This is an important factor in choosing a language. The easiest language to learn also has a lot of resources available. If you go to a bookstore or search at a library, can you find books, dictionaries, and phrase books in your target language? If you look on-line, can you find websites, podcasts, and other material in the language? If you search around, can you find movies dubbed in the language? Finally, are their native speakers to practice with? There are all important things to consider. The easiest foreign language to learn will be one where you have access to native speakers and a good selection of reference material.
Finally, do you have a strong motivation to learn the language? For instance, why would you want to devote a lot of time to learning Korean if you have no interest in ever going to that part of the world? But if you are enthralled by the sound of Italian, love pasta, long to explore Cinque Terre, and fantasize about becoming romantically involved with an Italian, then choosing Italian is probably a no-brainer. The easiest foreign language is one that you really want to learn.
Having strong motivations on many different levels to learn a language is crucial to your success. It will keep you motivated, keep things fun, and keep you going through the rough patches. Language learning is like any other skill. You have to work at it. Like anything else, you have to develop strength in it. Learning one language will in turn make it easier to learn the next language. Start with an easy language and and build up to the harder languages later on.