I'm not a linguistics expert at all. Just interested in the subject.
Doesn't include creoles, pidgins, or languages like Bislama that are English-based. Also includes languages that may not be that easy in general, but are considered the easiest language to learn from a family of languages. Example: If you want to learn Slavic languages, the most accessible are considered Bulgarian and Macedonian, so it may be easier to start there than notoriously tough languages like Slovak or Polish.
Most of the information on constructed languages comes from Wikipedia research. Most of the information on natural languages comes from the blog of journalist and linguist, Robert Lindsay. His system of ranking language difficulty is more detailed than that of the Defense Language Institute or the Foreign Language Institute. He ranks languages on a scale of 1 (very easy) to 6 (extremely difficult). Languages like Navajo and Abkhaz are level 6 while Lindsay gives English a 2.5 with the caveat that English can be deceptively complex after the basics.
This list may change based on comments, new research/new information. Thanks for reading.
http://www.ranker.com/list/the-easiest-language-to-learn-_for-native-english-speakers_/admiralcrunch,
Spanish Language
According to linguist/blogger Robert Lindsay, Spanish, along with a dialect of Italian called "Neapolitan" are considered the most accessible romance languages. Neapolitan conjugation, according to Lindsay, is simpler than standard Italian.
Spanish also is one of the few natural languages considered to be more or less phonetic.
Esperanto the International Language
The most popular constructed language and the first that has native speakers, as some Esperantists have raised their children to speak Esperanto as their first language. Esperanto has generated a large number of reforms to help simplify the language further. One debate is that Esperanto is "too regular" and therefore is not natural. (An example is that all nouns end in "o".) I'm not sure how important that is, but opponents of Lobjan apparently feel a language that "feels unnatural" may complicate communication and "mental translation" for speakers who aren't fluent.
Esperanto (and Ido) uses some Slavic vocabulary along with Romance and Germanic language vocabulary, also making it less accessible to English speakers.
Esperanto was constructed in the late 1800s.
Lingua Franca Nova
A modern constructed auxiliary language based on romance languages. Therefore likely easier than other "conlangs" because of the natural sound and the high number of English cognates.
Lingua Franca Nova can also be written in Cyrillic letters.
Lojban
A recent conlang based on logical clear communication and cultural neutrality (based on Mandarin, English, Hindi, Spanish, Russian, and Arabic). Proponents consider it the most optimal full auxiliary language if the objective is unambiguous/clear understanding. Detractors say its unnatural and better as a tool for computers/artificial intelligence.
Ido
A streamlined and simplified reformation of Esperanto developed in the early 1900s.
Interlingua
Similar to Lingua Franca Nova, but possibly more complex. Since Lingua Franca Nova is based on existing Romance creoles, it's considered simpler. Interlingua's regularity makes it intelligible to speakers of romance languages.
According to Wikipedia, an experiment showed students of Interlingua could translate complex Spanish texts more easily than students who had studied Spanish, indicating that the regularity of Interlingua helped students "grasp the mechanisms of language quickly."
Both Lingua Franca Nova and Interlingua may be more useful to dyslexia sufferers than English or most natural languages. According to Skeptics, Spanish speakers and speakers of other languages that are phonetic or nearly phonetic (like Finnish, Hungarian, or Bahasa) have many fewer instances of dyslexia and problems are much easier to correct at an early age. Because Lingua France Nova and Interlingua are free of irregular words and diacritical marks, they would be even more beneficial.
Toki Pona
Toki Pona isn't considered a full auxiliary language, but a minimalist language to help create harmony among people who wouldn't otherwise be able to understand each other at all. Toki Pona was developed by linguist Sonja Lang and is based on Taoist philosophy.
Novial
Otto Jespersen, a trained linguist, constructed this language in the late 1920s as a way to improve on Esperanto's "unnecessary or arbitrary" aspects. Novial is also based on Germanic and Romance languages which makes it easier than Esperanto or Ido for English speakers unless they already know a Slavic language.
Lingwa De Planeta
A modern, highly accessible constructed language that is based on several languages worldwide (Arabic, Chinese, English, French, German, Hi ndi, Persian, Portuguese, Russian and Spanish) instead of possessing a solely Eurocentric vocabulary. Since there will be fewer cognates with English, Lingwa de Planeta and Lojban might be slightly more difficult to acquire for English speakers than Ido, but Lingwa de Planeta especially may be more useful as a worldwide communication tool.
Afrikaans
Afrikaans is a language that evolved from Dutch but also has influences from African languages (Bantu and Khoisian languages) and Portuguese. The grammar is considered simpler than other Germanic languages including English and Dutch. For example, Afrikaans has nearly no irregular verbs.